Google Pagespeed Insights has bugs with showing preview images when a server supports Brotli compression.
Additionally, they're oddly sending "Accept-Encoding: gzip,deflate,br" for plain HTTP requests, not just HTTPS requests. This isn't very useful since no browsers currently intend to support Brotli over plain HTTP. However, that's probably unrelated to the preview bug since the previews aren't working over HTTPS, either.
Here's a site that supports Brotli that doesn't use ServerPilot (at least as far as I know).
https://community.centminmod.com/
It also has a blank preview image on the Google Pagespeed Insights:
Please feel free to share this info with the Google Pagespeed Insights team.