I'm seeing the same problem, but it's not limited to fullscreen (I rarely run in fullscreen mode). Mail is hidden/minimized and out of the blue it pops on the middle of the screen. I have also noticed that since 10.14.5 it keeps on "refreshing" the preview pane: it clears and then displays the same message again.
The only change I did is to add 2 gmail accounts that were not previously in use. I will try turning them off and see what happens. And yes, it started (the anoying refresh) with the GMail fix.
The only change I did is to add 2 gmail accounts that were not previously in use. I will try turning them off and see what happens. And yes, it started (the anoying refresh) with the GMail fix.