Could it be related to the theme being used?
I’m on UM 22.04 and tried all the installed themes and could only get the buttons to turn dark in dark themes. Windows might be different. If custom maybe try switching to another theme or default.
Just a possible guess.
Also is there another version of English as choice in your system?
Might be interesting to find out if it is repeatable for OP.
Also
Which window, main or the dialog?
On my UM22.04 KiCad 6.0.6 I can resize main window but can only resize dialog window larger. Actually in my system have similar (not related to above) issue where at times with a file folder open unable to rename files which is cured by slightly resizing window.(bug)