@aeris Please provide the following info:

1) Confirm the device is locked as expected
2) Device model
3) GrapheneOS version from Settings > About > Build number
4) Version code of the app shown at the bottom of Settings > Apps > App name
5) Does GrapheneOS show a notice about it using the Play Integrity API? If it doesn't then the app isn't using it so that's not the issue.

Try the per-app exploit protection compatibility mode to see if it's a memory corruption bug or similar issue in the app.