Reported by our licensee Gungho.
Reproduced in 4.21.2, win10
Open SamleLightMap project with vanilla 4.21.2
Download project here [Link Removed]
Reimport the assets in ReImportMesh.zip to /Game/Asset/Common/Stage/S020_Stg_UK/StMsh/Building/
Build the lightnings (Preview).
Notive the areas circled in red in the 2 png after the lightnings have been built.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-69501 in the post.