The app worked without issues until a few weeks ago. I used it for a year. It was not broken. GrapheneOS is just AOSP Android, optionally with Google Play Services.

My take is that they were trying to block rooted phones and/or custom ROMs of questionable origin and GrapheneOS just became collateral damage because all these companies do go the minimal route of using Play Integrity. GrapheneOS supports remote attestation through AOSP APIs, in fact, they have a page about it.

I think it's worth letting this be heard. GrapheneOS has > 400,000 users and is rapidly growing. Breaking things is not going to affect 5 people anymore, but thousands, ten thousands or hundreds of thousands, depending on what the app is.