Hey bubkins,
Thanks for your question about the message you see within the group conversation that you have. I'd be glad to see how we can get to the bottom of this and potentially getting it to go away.
You mentioned that this has been going on for a while now. Is this the only conversation in Messages, either standard or group, that this happens in? If so, does adjusting the name of the conversation cause the error to no longer appear? Are you the only person in the chat who sees this error, or does it affect all members of the group conversation?
If updating the name doesn't cause the error to go away, you might try recreating the group chat, which should stop the error, if no other conversations share this issue. More info on that in Send a group text message on your iPhone, iPad, or iPod touch.
Hopefully that helps.
Take care