Never seen that, and the string "netbox.backend" doesn't occur anywhere in the Netbox source code.
Are you talking about the URL displayed when hovering over a racked item, i.e. at /dcim/racks/<N>/ ? That works correctly for me.
I think this is most likely a bug in your http proxy (nginx/apache) configuration. Does it forward to http://netbox.backend:xxxx/ by any chance? In that case, you need to configure your proxy to keep the original Host: header. I use Apache2, and have the setting "ProxyPreserveHost On".