haakon 100%, not sure why I didn't think of that. Staged deployments are common for large software projects.
What annoys me currently is that it's hard to tell whether this is BankID who decided to enforce Play Integrity in strong mode, banning non-GMS-licensed OSs (the notification about Play Integrity API usage is suspicious, although BankID was definitely using the API long before 4.7.7, and the app still worked fine except for the minor passkey feature), or whether it's an app bug that is somehow triggered on GrapheneOS but not stock PixelOS.
I think I'll try to give bankid-app-feedback@bidbax.no a shot. And I'll give the app a one-star review on Play Store. I'll contact my bank also if the behaviour occurs in the next version, but I'm not looking forward to going back and forth with their support; my bank has told me in the past that GrapheneOS is not "supported" and that I'll have to "uninstall" it, but that was before 4.7.7 so they were clearly not referring to the app blocking GrapheneOS.
By the way, if you're making a Play Store review, make sure to not include any URLs, because Play is known to block/shadowban reviews that contain links.