File chooser dialog opens on xsession/screen 0 when Okular is run in (tiger)VNC session on port 5901

[zkeng]

Computer with a TigerVNC server session (a second xsession) running on port 5902.
Okular starts and works fine in this VNC session.
But any file chooser operation (Open, Save as) will open the file chooser dialog on the physical screen (not in the VNC session).
This issue seems to be unique to the KDE flatpak apps (verified in both Okular and Gwenview) because I run many other flatpak apps (such as Gimp, Inkscape) and none of them has this problem. I attach a screenshot with Okular running in VNC on port 5902 but the file chooser dialog has popped-up in the VNC session running on port 5900 (5900 is the same xsession that is running on the physical screen).

[zkeng]

I first created a report here:
flathub/org.kde.okular#150
But as it turns out other KDE flatpak apps are affected by the same issue. I installed and tested Gwenview flatpak as well and it suffers from exactly the same issue.
I am not using KDE as my DE (custom desktop with openbox WM), so flatpak apps default to using the Gtk/Gnome file chooser and it seems to always pop-up in the xsession running on the physical screen.