This seems to be caused by the following:
Create a struct with a custom NetSerialize function, which serializes at least an actor reference property as well as some other property, such as an int counter. Implement an OnRep function for this struct.
As the counter goes up, set the actor reference property (note the value of the counter).
Have the referenced actor go out of relevancy, then become relevant again.
Expected: the OnRep will be called for the struct, with the now mapped reference as well as the current value for the counter.
Actual: the OnRep will be called with the mapped reference, but the counter will be set to the value it was equal to when the reference was first set.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-183263 in the post.