If the JavaFX app is run locally on the computer the correct value is returned.

If the JavaFX app is run over Microsoft Remote Desktop (the app from the App Store) the Screen.getPrimary().getDpi() returns 0 at any time in the app (not just initially).

It does not matter if "Retina mode" is used in Microsoft RDP.

I guess that the RDP connection is creating its own virtual screen or something, because what I see on the Mac (over RDP then) is not what is shown locally on the screen of the computer, which just shows the lock screen.