There must be something else going on in your code because I cannot confirm what you're saying. Reducing the main form's size after maximizing doesn't throw an error here.
Where do you position the tabs of the tabsheet? At the bottom by any chance?
Indeed there must be something else going on… But I couldnĀ“t reproduce it either with a clean project, just in my project that is populated with a lot of controls… so, it must be difficult to find.. but anyway, I believe that routine is flawed because it never checks for the rect size before applying the fill