Are you testing it in a standalone window? Toolbars and such will reduce the viewport resolution when you run it in Selected Viewport. My viewport is 1513x963 (out of a standard 1920x1080) for example, and distorts UMG badly. The issue disappears when run in a New Editor Window (PIE).
If the user decides to scale the resolution to something weird, they’ll need to deal with the consequences. Also, have a look here for more info: