This has successfully been resolved using the workaround outlined here: Multicast not called in replays - Programming & Scripting - Unreal Engine Forums
Note that this may make it impossible to make a real-time multiplayer game (which is fine for my situation), since the actor will now always access the DemoNetDriver, which is solely for replays as I understand.
It would be great to get some clarification from a dev though, as to why DemoNet behaves the way it does under certain circumstances.