I seem to recall that once you leave a conversation yourself you can't even be invited back into it. I'm not sure if that's a bug or intended though. If you don't need to see the contents of the old conversation you can recreate it inviting all the necessary people.
It seems intentional: https://xenforo.com/community/threads/rejoining-a-conversation-you-have-left.26423/ The only fix seems to be via the database, which I don't have access to.
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