I am facing the same problem but using telegram https://community.n8n.io/t/issue-with-loop-using-telegram-node-and-basic-llm-chain-to-modify-the-prompt/89438?u=carlosmprz.
I have a similar loop trying to modify a prompt after a telegram message using send and wait to respond, after reads you i start to think this is kind of bug, because after saw a lot of video tutorial, all do the same and works fine for they.
First i connect directly the llm basic chain to telegram and didn’t work, i use the SET no approach and didn’t work too, apperently is related with back to a node.
I hope someone knows something about it. For the moment i’ll keep testing