At first, I thought it just happened when I installed the application. But nope — any time I open Teams, it will re-insert itself in my list of login items. This happens no matter how many times I have manually removed it.
I think it even happens more than once per launch because I recently launched it to join a call and then immediately removed it from my login items before the call even started. But after the call, there it was again in my login items.
I have never seen an application behave this way that was not widely considered to be malware.