It seems mob spawn, but you have to update their spawning blocks by moving towards or away from them. Standing still will not spawn mobs in the nether at all, and not in the overworld either.
I can confirm that this happened on my realm server as well. My brother lost items from his chest when we updated the map to 1.13.
Håvard Orskaug
It seems mob spawn, but you have to update their spawning blocks by moving towards or away from them. Standing still will not spawn mobs in the nether at all, and not in the overworld either.