I have this same issue, only when i go from a “standard” resolution to a non-standard (my example is 5760x1080)
i get some really weird UMG things on the wrong screens (3 screens stretched)
example: half my right side menu items are on the left most screen. partial remaining items are in the middle screen or, in the right place. I think it may have to do with anchors … guess i will have to double check