Hello wmbuRn,
This is a known issue (UE-4659) that has been submitted to the developers for further consideration. I have have provided a link to the original issue below. You can check there for future updates. I will be sure to bump the community interest for this issue. Thank you for your information and time.
Link: 4.5 UMG Widget Scale not working correctly! - UI - Epic Developer Community Forums
Make it a great day