BankID FAQ:
Some Android devices lack technical support, or so‑called “key attestation,” which is required to enable biometrics. This is when Google verifies the security of key management. For example, if you modify the phone yourself, it can happen that Google is unable to vouch for it.
https://bankid.no/hjelp-bankid-app
This FAQ statement feels incomplete, and it suggests that I should keep sending them feedback every few months about adding support for GrapheneOS.