I'm not sure if this makes a difference but do you see that problem with dialogs...

I'm not sure if this makes a difference but do you see that problem with dialogs (resources + DialogBox) rather than basic windows?

If I remember & get a chance later I can try to make a minimal example.

Edit: Based on this comment [1] I'm guessing you're on something like Windows 8.1 rather than a recent version of Windows 10 or later.