Generate Lightmap UVs fails very frequently on FBX imported meshes.
Found in 4.22 CL# 7053642, 4.23 CL# 8386587, 4.24 CL# 8971176
There will be a dense cluster of overlapping lightmap UVs in the top left corner and when lighting is built it will give bad results
Proper light map UV is generated reliably
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-80751 in the post.