Wither skull afk farm; tame wolf kills, rare drops only with owner present

Haven't tested on any other version, but it seems to be a pretty specific issue.

Set up an afk wither skull farm. Works quite well with the tamed wolves owner present in the nether.

However, if another user tries to afk at the farm it produces no skulls. 

Owner is still in the world, but not the nether dimension.