Mobs killed by lava floating above a bottom slab do not drop anything

Mobs killed by lava floating above a bottom slab drop their items in the block above the slab, where a hopper below the slab cannot suck them before the lava destroys them. Previously, the items were dropped on top of the bottom slab and the hoppers would suck them before they were destroyed. If items are thrown into the space above the slab and below the lava, the hoppers will suck them, so the problem is that the drops are being placed higher up than they used to be.