I can reproduce the issue (Win 10, Laz trunk, fpc 3.0.4) for a form with initial WindowState = wsMaximized, but only when the bsMaximize style is removed from the BorderIcons: the statusbar is covered by the TaskBar of the desktop. However, Delphi has the same behavior, therefore I think the issue has its origin in Windows.
You can prevent the statusbar of the maximized form being covered by the TaskBar when you switch the FormStyle to fsStayOnTop.