OK, now I can reproduce it (Win11, Laz/main+fpc3.2.2 (32bit), Laz 2.2.4+fpc3.2.2 (64bit), Laz 2.0.12+fpc3.2.0, Laz 1.8.4+fpc3.0.2, Laz 1.4.4+fpc 2.6.4)
I click the same spot of the groupbox not covered by a control about 10 times, then click on an empty spot in the taskbar, and the controls in the groupbox disappear (the groupbox border remains). A click back into the groupbox is not necessary.
In the Laz versions older than 2.0, the DoubleBuffered propery is not published and is off by default. But in the default state, and when I activate it in FormCreate, I get the same behaviour essentially, but now the areas in the groupbox covered by the controls are painted as black rectangles. When i move the mouse over these areas (without any button pressed) the controls re-appear. This happens also in the newer versions in which the black rectangles do not appear.