The issue occurs if a non-replicated actor is attached to a replicated scene component on the client, and then the same is done on the same scene component on the server. The attached actor does not need to be the same on both the client and server as long as they both are not replicated.
Attach a non-replicated actor to the pawn's replicated RootComponent on the client, then do the same on the server.
Expected: The server will replicate down AttachChildren with a null entry, and OnRep_AttachChildren will correct this using ClientAttachChildren.
Actual: The repnotify function is not called, so the AttachChildren with the null entry remains on the client, and the originally attached actor is no longer part of the array.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-107241 in the post.