Today, I had some additional behaviour: I typically close Brave with open windows. After reboot on the next day, I will have the same windows with the same resize-issue.
Today, that naughty window has a bigger minimum size.
What’s more, it was stuck on the uppper edge of the screen. When I dragged the window down (not resizing but changing the location on the screen), the window always snapped back. That behaviour went away a whil later after I had closed a few tabs.