VCL custom TGraphicControl messes up only on a remote desktop - c++

Question

In a C++ application (using C++Builder 10.3 and VCL), I use a custom class derived from TGraphicControl to draw a custom user interface and a small plot of changing data.
Everything works well in Windows 10 and Windows 7, but if I execute the application in a Remote Desktop session (via a native Windows service) with each redraw the graphics are messed up more.
The problem starts with disappearing elements and wrong LineTo endpoints, but sometimes the elements appear even in a different dialog instead.
Also, switching on a PC from direct to remote access causes dynamically inserted VCL TComboBox elements to get invalid parents and owners, followed by an error message that the TComboBox class is unknown.
I'm using ClientRect in my code. Is this perhaps the problem when working in remote sessions?
Code where I see the problem (it's a bit long and I have trouble making a minimal example without omitting possible causes of the problem)
cpp: https://pastebin.com/vpk3De7J (painter at line 228)
header: https://pastebin.com/3pLy4dEY
class CustomTrgDrawings : public TGraphicControl
{
private:
void __fastcall Paint(void);
public:
__fastcall CustomTrgDrawings(TComponent* Owner);
bool updating; //stops painting when true
};
__fastcall CustomTrgDrawings::CustomTrgDrawings(TComponent* Owner)
: TGraphicControl(Owner)
{
updating = true;
}
void __fastcall CustomTrgDrawings::Paint(void)
{
if (!updating)
return;
Canvas->Brush->Color = TColor(0x220022);
Canvas->Brush->Style = bsSolid;
Canvas->Pen->Color = TColor(0x222222);
Canvas->Pen->Style = psSolid;
Canvas->Pen->Width = 1;
Canvas->FillRect(ClientRect);
}
//---insertion in a vcl form
void __fastcall TMyForm::FormCreate(TObject *Sender)
{
//uses TLabel *myLabel as container
CustomTrgDrawings *drg = new CustomTrgDrawings(myLabel);
}