Lazarus 2.2.4 (rev lazarus_2_2_4) FPC 3.2.2 x86_64-win64-win32/win64
I've run and tested, strange thing is, when I build on my own, all works like it should, importing and running your demo shows me same behaviour.
It seems like, when "TGroupBox" is the only control on form, something shitty happen at repaint/refresh/update.
First thing I've changed was to put a panel inside the GB with alClient and moved all of your controls on it. That reduced the error but not stopped it.
Second, I've put a panel alClient on form on moved GB alClient inside. That reduced the error but not stopped it.
Third, I've put another panel alBottom on form (outside of above panel/GB). That almost reduced the error to zero but not stopped it.
I am clueless what there is going on. I try to investigate that deeper. To me it look like a bug.