When you assign a Material to a Multi-Material Skeletal Mesh via the Mesh's Level Viewport Details Panel, you will lose access to all the other material Elements after the first assignment. For instance if your mesh has 5 Elements and you assign a material to Element 2 than Elements 3 and 4 will disappear.
This problem does NOT occur on Static Meshes with Multiple Material Elements.
1. Open QAGame Editor
2. Add QA_Assets/SkeletalMesh/Main_Player_Export to the level
3. From the Mesh's Level Details Window, add QA_Assets\Materials\Test_BlueMaterial to Element 0.
Access to Elements 1 through 3 disappears
Still have access to all material slots
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-5746 in the post.