How do I join back a conversation in Messages?

I left a conversation not realizing I was not able to rejoin at will. Didn’t think to check first. I am still receiving messages of the conversation thread, but cannot respond or make any other changes. I’ve deleted the thread. I’ve started new ones. And I’m still not able to rejoin. They have tried adding me back on their end as well and still no luck. Does anyone have an explanation or fix for this?

I find it very odd that if you choose to leave a conversation you still receive the messages. You leave because you don’t want to receive them anymore not because you don’t want to ever be able to respond anymore. Doesn’t make sense. If you choose to leave it, you should be able to rejoin it.