1. Create a widget with 2 buttons, one on top-left, another on botton-left
2. Add this widget to viewport at BeginPlay
3. play in standalone game and fullscreen
4. Test mouse hover and click
Result: Upper part (red rect) of the bottom button can't response mouse hover and click
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-160253 in the post.