An unsolicited 'You're welcome' is a textbook example of a bid for covert control of an unspoken societal contract.
It is 100% okay to respond with, "That's a narcissistic move. I don't want anything further to do with you."
When the Queen of Swords enters the chat, emotions evaporate. Lean into that energy. Hard.
Shut that shit down immediately and get out of there. Playing mindgames that usurp your agency is malicious. Run. You don't owe them an explanation.
If you can't just cut this person off, start planning an escape. I'm not qualified to help in those situations, but there are people who WANT to help you. Look into it. You are worth saving.