Based on what you're describing, it sounds like their app now requires a Google certified OS, which GrapheneOS isn't.
They'd need to not do that anymore, or optionally whitelist GrapheneOS. There is a guide meant for developers to implement this for their apps, though it is unlikely they'll do it, I'm afraid:
I wan to be clear that I don't know for sure that this is what's happening, it just sounds the most plausible explanation based on what you're describing.
The only other thing I can think of is to go to Settings > Security and disable Secure Exec Spawning, reboot, and try again.
If it still doesn't work, re-enable it and reboot.