It looks like this only happens to multicast RPCs that are outgoing from a server. Client RPCs still show up as expected, and both incoming and outgoing RPCs appear properly in a client's session.
Use networking insights on a server instance that is sending a multicast RPC.
Expected: the RPC and its parameters would show up in the packet content panel
Actual: the RPC and its parameters are not visible, and its name is not listed in the Net Stats panel
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-129870 in the post.