Import a staticmesh using more then one material
Change one or more section material slot index in the sections array of LOD 0
Change the LODGroup from "None" to "Deco"
The sectionInfo map of LOD 0 will be reset in the section index order.
The sectionInfoMap of LOD 0 should not be touch when changing the LODGroup, since it will be use to generate the LODs. Also for LODs we want to remove the sectionInfoMap entry not reset it to sectionIndex.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-81507 in the post.