Widget Blueprint → Add a Border with a Textblock in it.
Textblock → Font → Outline Settings → [Outline Size = 1] & [Outline Color = White]
Now from your Border change the Content Color and Opacity.
The outline isn’t touched at all.
Hello Absolute Reality,
This appears to share the same root cause as a known issue. I have provided a link to the public tracker. Please feel free to use the link provided for future updates.
Link: Unreal Engine Issues and Bug Tracker (UE-40691)
Make it a great day