In this example, the multicast event is on the player controller, which does not get replicated to other clients, so that's why the event doesn't fire. Placing the multicast event on either the level BP or character BP works correctly, so closing as by design.
Using a multicast function does not call the function on both client and server. The functionality in this reproduction project can be found in the BP_MyPC blueprint.
This occurs in 4.19
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-63720 in the post.