OSX Firemonkey full screen modal form problem

I have a main form and a modal dialog box that is opened with a button on the main form. Both forms have ShowFullScreenIcon property set to false. When I compile to OSX the Full Screen Icon is showing (and functional) in the compiled application. When I click the full screen icon, the main form becomes full screen. Now when I click on the button to open the dialog box it shows as tabulated full screen. Selecting the Main form tab causes the application to become unresponsive requiring a Force Quit operation. I am running Firemonkey 10.3 Rio and macOS High Sierra. The bug was reported to me by one of my clients who is running my application on Mojave. The issue is extremely easy to reproduce.

I notice that this point has been raised previously by another member but that thread has been locked so I cannot see any response to it.

Is this bug or am I missing something?