A quick Google search on the problem suggests that it's relatively simple for Mojang to fix by using the ExtendedExecutionSession API in the method that is the UWP app's entry point.
Of course, things are rarely as simple as a quick Google search might suggest.
Whilst we're waiting for a fix, note that you can quickly switch in and out of full screen mode by pressing
SHIFT-WIN-ENTER
.Also, if you absentmindedly switch to another app from full screen mode, if you're quick you can
ALT-TAB
back into Minecraft without the server disconnecting you.It's far from ideal, but better than running in windowed mode constantly.
Regarding a fix, I'm wondering if it would be possible for the devs to automatically switch to windowed mode when losing focus to prevent it from being suspended. Might be an easier workaround so long as a 'lost-focus' event fires before the task is suspended of course...