Adding a retarget source to a skeleton would create a hard reference from the skeleton to the mesh(retarget source mesh).
1. Open the attached project and check UE_Mannequin_Skeleton in Reference Viewer, to see the skeleton has a hard reference to Owen
2. In Retarget Manager, remove the retarget source to Owen to see the hard reference is removed
3. Adding back a retarget source would create a hard reference again
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-48570 in the post.