Before assigning RefSkeleton, FSkeletalMeshMerge::MergeSkeleton calls BuildSockets->RebuildSocketMap->FSocketInfo->RefSkeleton.FindBoneIndex, which incorrectly sets the SocketBoneIndex.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-78645 in the post.
| 4 | 
| Component | UE - Anim - Runtime | 
|---|---|
| Affects Versions | 4.23 | 
| Target Fix | 5.2 | 
| Fix Commit | 18225313 | 
|---|
| Created | Aug 9, 2019 | 
|---|---|
| Resolved | Jan 24, 2023 | 
| Updated | Feb 14, 2023 |