Consulted a friend who teaches Unreal and finally came up with a solution!
The problem is with the way the UV’s are assigned when converting the geometry brushes to a static mesh.
Fix:
**- Double click the created static mesh in the content browser
- Change lightmap coordinate index to 1 (search or scroll down to general settings in the details window)
- Change minimum lightmap resolution in LOD0 to something higher (e.g. 512)
- Change lightmap resolution in general settings to something higher (e.g. 512)
- Apply and save your changes
- Build the lighting again**
You can use any resolution you’d like, though 512 should be more than enough for whiteboxing.
We tried shifting around the Source and Destination lightmap index first, but that wouldn’t budge at all. So ignore that, leave it on its default and just change the Lightmap Coordinate index in the general settings.
I hope this helps someone else that’s facing the same problem 