Oh man. It's not just Apple. I've had months of RCS not working on GrapheneOS, and have no idea who to blame. The first time it stopped working, I fixed it by switching carriers (AT&T -> T-Mobile). Maybe I'll try switching back! Or maybe I'll switch back to an iPhone and give in to iMessage. :(
It worked for me on GrapheneOS for quite a while, but a couple months ago things started breaking and I no longer have it enabled. There's an absolute behemoth of a thread discussing the issue, and unfortunately it's still active which I assume means I'm not safe to enable it again yet. If you want some light reading to help put yourself to sleep: https://discuss.grapheneos.org/d/1353-using-rcs-with-google-...
Honestly at this point, untangling my group chat mess was such a headache that I'll never turn RCS on again. I need to have 100% confidence that my messages are received and sent, and Google has forever broken that trust re: RCS. I managed to coax most of them over to alternative platforms, but I can't subject my poor grandmother to that headache, so it's SMS/MMS going forward for me.
I had the same issue, with Google Fi! The only thing that briefly resolved it was swapping my number over to an older phone running stock android. Stopped working again when I switched back to my other phone. I just ended up turning it off entirely, but it irreparably broke a few group chats I was in.
