Iris - Fast Array ItemMap is empty in PostReplicatedAdd

With Iris enabled, the ItemMap is populated correctly only on the next frame, but it is empty during the callback.
This differs from the behavior of the current system, where the ItemMap is populated before PostReplicatedAdd.

Create a fast array with a PostReplicatedAdd callback.
On the server, add items to the fast array.
With Iris enabled, observe that the fast array's ItemMap is empty when the client calls PostReplicatedAdd.
Without Iris, the ItemMap is populated as expected.