This... sort of worked for me?
There are three apps for teams on my computer. (I have now uninstalled teams (classic) since it won't even open on my system anymore.)
I used the "reset" button on both the Teams (new) with the white icon in the foreground AND the Teams (new) with the purple icon in the foreground. After using reset both times, there was still no change. I then used repair on both versions, and still no change.
However, I had to go through and re-submit some of my preferences/settings, and noticed that the search bar at the top of the screen no longer has a left arrow to shrink. After clicking away from settings and messing with the search bar, the collapsible bar finally collapsed.
I'm not certain which part of this process fixed it, or if it was a combination of futzing with all of them.