Try flashing a clean image. I suspect upgrading from LibreELEC will not work properly.
Sorry, I wasn't clear enough. That's my fault. By upgrade, I meant that I've removed the USB running LibreELEC and put in a new USB with RetroELEC. There is no HDD, so it's all run off the USB for now. So it is a clean install.
The issue seems to have something to do with the version I was using. I've tried again today with the newest generic build and it works. Whatever you've changed seems to have fixed it. Thanks!
RetroELEC-Generic.x86_64-9.2-devel-20200226025821-28eddc2 = works
RetroELEC-Generic.x86_64-9.2-devel-20200214162808-28eddc2 = Boot loop