800 is the total screen height not the 'usable' screen size!
So your the buttun bottom is below the usable screen bottom.
If you set, in the Designer, Show Title to False and Full Screen to True the button will fill the whole screen.
Please, Klaus, see the image well... I set Show Title = False and Full Screen = True.
I'm thinking that the problem is also related to the Grid setting.
After many tests, setting it to 10 then to 50, closing and opening the Project again (not just the Designer) the button sometimes will be completly out of Designer or half inside it. Randomly
[correction: a label I added will be outside the Designer, not the button; but the error is there, anyway]
The bar at the button cannot be removed. So even in full screen you lose a few pixels (the abstract designer removes 48 pixels for that).
There is a bug here and it is related to the emulation of Android 4+ phone devices which don't remove this offset. However it is not really important as you should not rely on the exact size. There are so many devices that you shouldn't try to match the exact values of any device.