UMG widgets that are scaled negatively in order to mirror them do not render properly and have a strange green gradient. [video]
There was a report of this back in 2014, but it doesn’t seem to have been fixed.
EDIT: After testing it a bit further, it seems that the green artifacts don’t actually effect anything, and the green artifacts go away when the widget is deselected, however progress bars go completely invisible when mirrored.