User reported an issue of a mesh not being visible when a project has multi-level blueprint inheritance. After reproing, I believe the root cause is DefaultSceneRoot of the grandchild BP class not correctly inheriting Mobility of the grandparent class, or that Mobility being modified during loading. This in turn affects the attachment process of the DefaultSceneRoot child StaticMeshComponent. UDN case linked.
Observe the DefaultSceneRoot's mobility in BP_C's has changed to Movable, rather than inheriting Static.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-163285 in the post.