When using an older asset(4.10 or before) with a skeletalMeshComponent in a newer engine version, a reference for the original AnimBP remains after changing the mesh component's Anim Class.
1. Create a third person project
2. Duplicate ThirdPerson_AnimBP(in Content/Mannequin/Animations)
3. Open Blueprint editor for ThirdPersonCharacter
4. Change the mesh's Anim Class to the AnimBP dupliated in step 2, save and compile
5. Open Reference Viewer for ThirdPersonCharacter
6. The original ThirdPerson_AnimBP is still shown as referenced
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-42501 in the post.