Slime spawning on top half slabs/farmland blocks in slime chunk.

I am building a section of my base underground in a slime chunk.  It is a crop farm so I spawn proofed the entire floor with top half slabs, but slime are continuing to spawn on top of either the slabs or the farmland, which is wrecking my farm.  I don't think this is natural since they aren't supposed to spawn on any slabs or farmland blocks, right?