Hello and thank you very much for an example.
In the full version this problem does not occurs if TGroupBox is defined in the SkinManager.ThirdParty property or if used TsGroupBox control.
I see a problem with drawing and I'll try to fix it in the Lite version, groupbox must have just standard kind, but I can't guarantee it.
As a temporary solution for the Lite Edition I can suggest to place TsPanel control under GroupBox (with SkinData.SkinSectionProperty = 'MENUITEM', for a full transparency).