Hello!
First I know this bug has already been reported: (Jira UE-4659), UMG image starts to disappear after rotating - UI - Unreal Engine Forums . But it’s a known bug since october 2014 and nothing has changed since, so I would like to help you to solve this bug quicker.
First here a video showing the bug:
VIDEO
I think that I identified the main problem with the rotation of any element in UMG. The overflowing content of the element, out of the screen or out of the viewport is hidden for performance purpose I guess, but when you apply a rotation the hidden part is not computed again, finally then a part of your image disappear. The same thing happen in game when an element is a bit out of the screen.
I hope something will be done for this.
Have a nice week-end. Thanks!