DuxBellorum I think any intent to inform your bank about GrapheneOS and to ask them to include it in their safe os list is welcome.
A few days ago I complained to my bank about the same message you're getting, letting them know I'd probably have to switch to another bank if the app eventually did stop working on my phone. I mean, no payment validation is just impossible. I sent them the GOS guide for hardware integrity check linked above.
So far I haven't been able to login through the browser on my phone since it requires scanning a QR code to authenticate the connection... 🥲