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.
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I suspect it's not that mobs aren't dropping anything, but it's rather that lava is destroying items before hoppers can suck them. Hoppers should be able to suck items every game tick, and they should do so before items are destroyed. Slabs shouldn't be needed in the first place. See Hopper#Redstone component.

I suspect it's not that mobs aren't dropping anything, but it's rather that lava is destroying items before hoppers can suck them. Hoppers should be able to suck items every game tick, and they should do so before items are destroyed. Slabs shouldn't be needed in the first place. See Hopper#Redstone component.

Attached a test world.

Attached a test world.

I've been trying to decide how to update this. My original report was based on both a chicken farm and an iron golem farm where I had slabs above hoppers with lava floating above the slabs. In the chicken farm the slabs were necessary to create a half-block gap where baby chicks could survive but adult chickens were killed and cooked. In the iron golem farm the slabs were merely aesthetic, to hide the hoppers.
PHO suggested that the drops were being destroyed by lava before the hoppers could get them. This turns out to be correct, and I have updated the title and description correspondingly.
PHO also suggested that the slabs aren't necessary. This is true for the iron golem farm, but not for the chicken farm, because it works by hatching eggs in the space above the slabs. The half-block space between the slabs and the lava is big enough to keep the chicks safe, but when they grow into adults their heads reach into the lava which kills and cooks them. Without the slabs the chicks would be killed before they grew up (and even before that, the eggs would be destroyed before they could hatch).
I was able to redesign both farms to work despite this problem, so I no longer care about it, but it remains true that the original farms (whose design was taken from and, as far as I know, still works in Java and Console Editions) do not work as of 1.2.0.31, apparently because items dropped by mobs dying on bottom slabs are created in the block above the slab rather than on the slab's surface. This may be the intended behavior, but hasn't been identified as such.

I've been trying to decide how to update this. My original report was based on both a chicken farm and an iron golem farm where I had slabs above hoppers with lava floating above the slabs. In the chicken farm the slabs were necessary to create a half-block gap where baby chicks could survive but adult chickens were killed and cooked. In the iron golem farm the slabs were merely aesthetic, to hide the hoppers.
PHO suggested that the drops were being destroyed by lava before the hoppers could get them. This turns out to be correct, and I have updated the title and description correspondingly.
PHO also suggested that the slabs aren't necessary. This is true for the iron golem farm, but not for the chicken farm, because it works by hatching eggs in the space above the slabs. The half-block space between the slabs and the lava is big enough to keep the chicks safe, but when they grow into adults their heads reach into the lava which kills and cooks them. Without the slabs the chicks would be killed before they grew up (and even before that, the eggs would be destroyed before they could hatch).
I was able to redesign both farms to work despite this problem, so I no longer care about it, but it remains true that the original farms (whose design was taken from and, as far as I know, still works in Java and Console Editions) do not work as of 1.2.0.31, apparently because items dropped by mobs dying on bottom slabs are created in the block above the slab rather than on the slab's surface. This may be the intended behavior, but hasn't been identified as such.