There is a scenario where delta struct serialized fast arrays can replicate changes that introduce additional elements to the array, but without changelists containing the new data.
This needs further investigation to determine an isolated repro case. It is a regression against the original replication behavior of fast array serialization.
See linked UDN thread for more information:
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-118168 in the post.