I have 6a without issue, have you rebooted, is the FP gone from the lockscreen or the settings?
This in an owner or secondary user profile?
Discussion
Meant 'and' not 'or'
There are numerous reports across the web of this issue happening due to hardware requiring RMA and repair on stock it really isn't likely to be software/OS related and coincidental as we aren't receiving mass reports back and dev/staff devices are unaffected.
For example:
https://www.reddit.com/r/GooglePixel/comments/rty3gq/pixel_6_pro_missing_fingerprint_options/
ANOTHER reboot did the trick, the biometrics are back!
Only one user profile on the phone, after the update and reboot biometric disappeared from every menu.
Yeah this particular behaviour is an AOSP issue and not GrapheneOS specific.
If it failed to come back at all it would be hardware related.