Florian, I’m glad you were able to solve your problem. This afternoon, I found this, which I think is the formal answer to this issue: weird-set-master-pose-behaviour
EDIT. While I appreciate the staff explanation in the AnswerHub post I mentioned, I don’t think it’s a skinning issue. I think the extra bones for the slave component - in my case, a link of 3 joints for pony tail appended to the head joint which is at the end of bone hierarchy - just don’t get transformed correctly relative to the parent head joint. I see the following messages in the log for the 3 added bones repeatedly:
LogSkinnedMeshComp:Warning: GetBoneTransform : ParentBoneIndex(7) out of range of MasterPoseComponent->SpaceBases
It looks like the additional joints transform to near the root bone location and the verts follow and stretch to the floor after SetMasterPoseComponent is executed from the Character BP on Begin Play. I used GetAllSocketNames and GetSocketLocation do dump all the bone names and locations of the slave component to the log. Before SetMasterPoseComponent, the additional bones returned their locations. After SetMasterPoseComponent, the 3 additional bones locations simply returned {0,0,0}
Hopefully, I’ll learn something when I can spend some more time on it. As I’m a noob hobbyist, I hope I can find some correctly working examples we can learn from where slave components require additional bones.