Seems identical to what I had on Testing. This bug only appeared on my desktop computer that has two screens; it was fine on my laptop and VMs.
This should be fixed with Cinnamon 4.0.6 (or later), but as a temporary workaround, you can try to downgrade Muffin to 4.0.3 if you still have the old package in your cache. This is what I did personally.
From what I understand, this is an issue that is caused by a change in Muffin and it has been fixed with some changes in the code of the Windows list applet.