I’m using 4.9.2 of the Unreal Engine but this have been an issue since at least 4.8 with UMG. The Spinner widget is neigh on impossible to edit when you use it on the phone.
Here is a picture of my menu from my phone (Sony Xperia Compact Z1):
http://puu.sh/kv9Vs/da168c29fb.png
Here is what it looks like when you just touch the spinner box:
http://puu.sh/kv9Yd/2053aa4952.png
Still looks fine, but then look what happens when I press inside that box to edit the value manually instead of using the microscopic arrows that I can’t see any way:
http://puu.sh/kva0i/e919fa32c5.png
The dot disappears and trying to add it with the on-screen keyboard is just consumed instead of appended inside the box. Here is the more weird part: Some times when you edit the value inside that box it may stay behind the decimal point, but some times it won’t. It’s very inconsistent and almost completely unsuable on mobile.