Hoppers are ghost blocks for other players when placed on jukeboxes that are playing music

If you are playing with another player (in my case a LAN world) and you insert a disk into a Jukebox and then place an hopper on top of the Jukebox while it is playing, the other player will not be able to see the hopper and it will be a ghost block that will only appear after the player that can't see it clicks on it.