As You can see there in the attached example video, it is a problem in painting of MdiChild windows, when the windidow show, it appear a not skinned caption for a while.
I have made a test-app, based on your form, but can't repeat a described issue there.
Maybe you can help to repeat the issue? Maybe some code should be added? Please check it.
I do not know what code to add …
This window is part of a very large project that includes about fifty MdiChild windows and all have the same problem, which I noticed occur with the latest updates …