This is a really tricky one!
Multicast events are special animals. According to the Unreal Network Compendium, multicast RPCs have an optimization built in where they can only call so many times during a given update period. What seems to be happening is that since your event begin play is calling the multicast event on all instances of the pawn at once, it can only do so many at a time so drops them. If you add a delay between 1-3 seconds after begin play it should stagger the multicast calls, you should see what you expected in the first place. Hope this makes sense.