Hello ddwomo,
Thank you for posting in the Apple Support Communities. From our understanding, you are seeing a message that states "You left the conversation" in the Messages app, however, you haven't left the conversation. We are glad to assist.
According to the Apple Support article below, "If you see a message that says you left the conversation, then either you left the conversation or you were removed from the group message. You can rejoin a group message only if someone in the group adds you."
If you can't send or receive messages on your iPhone, iPad, or iPod touch
We hope this information is useful. Kind regards.