To anyone not interested in bending over backward for Chat GPT, https://www.futuretools.io keeps a pretty comprehensive, categorized list of AI tools, many of which use OpenAI's API, and many of them are customized to be be better tools for your specific use-case.
I downloaded the app as soon as it came out and I exclusively use Firefox. Are you perhaps logging using your Google account? I don't know if it could be that.
I use Mull and since it's Graphene OS, there is also vanadium (chromium based). I try fennec, but I think it's because Graphene uses a lot to block other google services.
Edit: maybe because I have vanadium webview instead of android webview
I had this issue with the Bose app and it took me a long time to figure out why adding any services would fail silently. Until I re-enabled Chrome, which suddenly fixed the issue.
As some people report the app is working on their phones without Chrome, is it possible you have installed an alternative System WebView on your phone, e.g. Bromite? Don't what some custom OS like Graphene use as default.