Suso

@Franco Caprile After checking, this seems to be what's causing the issue. Still, I don't thin this should be considered intended behaviour, since you're specifying a position in the array, I think the slot tag shouldn't be copied. It makes sense because the slot is stored inside of the item in the array, but I don't find it useful in any case. If you're trying to copy an item from a slot in a chest to another one, it would be much more convenient if the Slot tag adjusted automatically, since that would allow to use the "searching by tag" (Items[{tag:{}}]) much more versatile.

I'll update the description of the issue with hat we found out.

Just tested it on 18w30b, still broken for me

Seems to crash when a player goes under the world limit (Y=0) here. The world crashes permanently, even if the player that caused this to trigger is no longer there.

^{EDIT: Maybe the game is trying to spawn mobs on subchunk -1?}