I mean this is the whole reason GrapheneOS does not ship a rooted version that they attest to. I doubt they are explicitly banning GrapheneOS, what they are doing is only trusting Google attestation.
The reason some banks explicitly allow GrapheneOS is because its easy for them to also trust GrapheneOS attestation, because it is not a rooted OS, and sensible security teams in banks can see that its more secure than any other Android phone including Pixels.
I wish people would understand this better, its what gives GrapheneOS a fighting chance of becoming a legitimate open source mobile OS that is accepted as secure for consumer use and that is not controlled by Apple or Google.