Apologies for linking to a reddit post, but this issue has affected me it seems a couple others also running GrapheneOS.
https://www.reddit.com/r/signal/comments/1i0r0xs/app_issue_you_are_no_longer_in_the_group/
Essentially what's happening is I'm being removed from groups (even those I'm admin of) and the message just says: "You are no longer in the group". I still have access to the group, can scroll back through past messages, can view media etc. I just can't send messages to the group.
Anyone else here getting this issue?