You haven't re-created the described problem - Teams sets itself to auto-start again after you start it yourself. After all, it's very reasonable that you might want to join the occasional Teams meeting but not want it running after every boot.
I'm not sure if it's a particular version or environment that does this, but at the very least I can't replicate it on my home PC with Teams (personal). If I disable it in the task manager's autostart, it remains disabled if I start Teams. It won't even let me enable "Auto-start" in the Teams settings if it's disabled in the task manager.