Recent changes to the player controller's mute functions have resulted in the MuteList not being updated on clients, and instead APlayerController::ClientMutePlayer now just directly calls MuteRemoteTalker on the online engine interface. However, IsPlayerMuted still checks MuteList.
Mute a player on a client (APlayerController::ClientMutePlayer), then call IsPlayerMuted for that player on the same client.
Expected: IsPlayerMuted will return true
Actual: IsPlayerMuted will return false
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-169683 in the post.