See linked UDN for more details and a sample project reproducing the issue. The issue seems to be that the object's archetype doesn't have any changes made to the array in PostLoad, so the shadow state of the array isn't initialized with these starting values.
Create a replicated TArray property with a repnotify function on a replicated Actor.
Set the TArray's initial size and values in the Actor's PostLoad function.
Change the values of the array.
Expected: The changes will be received on the client and the array's OnRep will be called.
Actual: The changes are received, but the OnRep is not called unless the size of the array changes.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-119459 in the post.