A licensee reported that "Build" button causes "Validation failed" and breaks meshes in a Sublevel according to a prescribed procedure.
The process is quite complicated, but I can 100% reproduce this issue on 4.20/4.21.
So I posted this ticket.
I'm not sure what component is the most appropriate for this issue, would somebody please change Components if necessary.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-69842 in the post.