luigi Any idea how this could even happen (given that the bootloader is locked, there is no root, etc)?
Blastoidea Are the phones in fact rooted, or does the app just think they are?
That's just a one size fits all error message, GrapheneOS isn't rooted and doesn't support it either. The tl;dr is that GrapheneOS isn't certified by Google so if an app checks for that it fails, often throwing that error at you. Details here.
luigi Any suggestions for how to fix this?
Follow the troubleshooting steps here, failing that contact your bank and hand them this guide so they can start supporting GrapheneOS.