Mobs continue to spawn over the old mob cap but only spawn on lower land levels. If there is a new platform none will spawn on it. If a player is in spec mode on multiplayer, with another player in creative, then goes to creative mobs will no longer spawn for that person. If that player relogs then it's fixed.

Hard to explain the problem but example can be seen here. https://www.twitch.tv/videos/327273024?t=02h56m31s