Taking a look at the log file, it's not exactly clear to me what's causing this to happen. I've bolded a couple of lines that I think look suspicious. I've included the relevant excerpt below:
2021-02-19 21:40:42,073 [AWT-EventQueue-0] DEBUG ZestZapUtils - getElement My Selenium Juice Shop Unrecognised class: org.zaproxy.zap.extension.script.ScriptWrapper
2021-02-19 21:40:44,736 [AWT-EventQueue-0] DEBUG PopupMenuItemHttpMessageContainer - actionPerformed SITES_PANEL AJAX Spider...
2021-02-19 21:40:44,736 [AWT-EventQueue-0] DEBUG AjaxSpiderDialog - init org.zaproxy.zap.model.Target@2ca9de26
2021-02-19 21:40:52,528 [ZAP-AjaxSpider] INFO SpiderThread - Running Crawljax (with chrome): Context: Default Context
2021-02-19 21:40:52,529 [ZAP-AjaxSpider] DEBUG SiteMap - findChild Sites / http://kyles-juice-shop:3000
2021-02-19 21:40:52,529 [ZAP-AjaxSpider] DEBUG SimpleEventBus - publishSyncEvent scan.started from org.zaproxy.zap.extension.spiderAjax.SpiderEventPublisher
2021-02-19 21:40:52,529 [ZAP-AjaxSpider] INFO SpiderThread - Starting proxy...
2021-02-19 21:40:52,530 [ZAP-AjaxSpider] INFO SpiderThread - Proxy started, listening at port [36481].
2021-02-19 21:40:52,538 [ZAP-AjaxSpider] INFO Plugins - Loaded org.zaproxy.zap.extension.spiderAjax.SpiderThread$DummyPlugin@71364522 as a OnBrowserCreatedPlugin
2021-02-19 21:40:52,550 [ZAP-AjaxSpider] DEBUG SpiderThread - Setting up a Browser
2021-02-19 21:40:52,632 [Forwarding newSession on session null to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:52,632 [Forwarding newSession on session null to remote] DEBUG ZapProxySelector - Selected proxies for socket://localhost:20020 [DIRECT]
2021-02-19 21:40:54,045 [Forwarding newSession on session null to remote] INFO ProtocolHandshake - Detected dialect: W3C
2021-02-19 21:40:54,046 [ZAP-AjaxSpider] DEBUG ExtensionScript - invokeScript My Selenium Juice Shop
2021-02-19 21:40:54,074 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:54,290 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:54,509 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:54,721 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:54,926 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:55,135 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:55,342 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:55,549 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:55,755 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:55,960 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:56,170 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:56,384 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:56,595 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:56,804 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:57,014 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:57,221 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:57,226 [ZAP-ProxyThread-729] DEBUG ProxyThread - Timed out while reading a new HTTP request.
2021-02-19 21:40:57,425 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:57,630 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:57,839 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:58,043 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:58,250 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:58,458 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:58,671 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:58,883 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:59,088 [Forwarding getCurrentUrl on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:59,301 [Forwarding get on session 5e2ee4f9dddcabeb8ce9e18de40d3cc0 to remote] DEBUG ZapProxySelector - Selected proxies for
http://localhost:20020/ [DIRECT]
2021-02-19 21:40:59,328 [ZAP-ProxyThread-731] DEBUG HttpSender - sendAndReceive GET http://kyles-juice-shop:3000/ start
2021-02-19 21:40:59,330 [ZAP-ProxyThread-731] DEBUG HttpSessionsSite - No session tokens for: kyles-juice-shop:3000
2021-02-19 21:40:59,330 [ZAP-ProxyThread-731] DEBUG ExtensionScript - invokeScript My Juice Shop Session Management
2021-02-19 21:40:59,368 [ZAP-ProxyThread-731] DEBUG ZapProxySelector - Selected proxies for socket://kyles-juice-shop:3000 [DIRECT]
2021-02-19 21:40:59,371 [ZAP-ProxyThread-731] DEBUG HttpSender - SUCCESSFUL
2021-02-19 21:40:59,371 [ZAP-ProxyThread-731] DEBUG HttpSender - sendAndReceive GET http://kyles-juice-shop:3000/ took 41
2021-02-19 21:40:59,371 [AWT-EventQueue-0] DEBUG SiteMap - addPath http://kyles-juice-shop:3000/
2021-02-19 21:40:59,371 [AWT-EventQueue-0] DEBUG SiteMap - findAndAddChild Sites / http://kyles-juice-shop:3000
2021-02-19 21:40:59,371 [AWT-EventQueue-0] DEBUG SiteMap - findChild Sites / http://kyles-juice-shop:3000
2021-02-19 21:40:59,371 [AWT-EventQueue-0] DEBUG SiteMap - findAndAddLeaf http://kyles-juice-shop:3000 / /
2021-02-19 21:40:59,372 [AWT-EventQueue-0] DEBUG SiteMap - findChild http://kyles-juice-shop:3000 / GET:/
2021-02-19 21:40:59,409 [ZAP-ProxyThread-731] DEBUG HttpSender - sendAndReceive GET http://kyles-juice-shop:3000/styles.css start
2021-02-19 21:40:59,410 [ZAP-ProxyThread-731] DEBUG HttpSessionsSite - No session tokens for: kyles-juice-shop:3000
2021-02-19 21:40:59,410 [ZAP-ProxyThread-731] DEBUG ExtensionScript - invokeScript My Juice Shop Session Management