Golems spawing in slabs

the problem- I decided to make an iron farm from a youtube video ( https://www.youtube.com/watch?v=iFfmjIoGuhA&t=534s ) by BlendsCraft tv. once I was done I made sure to cover all exposed blocks with slabs to make sure there were no unwanted spawns. after this I noticed golems spawning in the slab and there were quite a few of them. some may have also spawned on beds which have no blocks underneath.

how to recreate- get a lot of villagers, put them on beds, make sure to have a work block for each bed. get a bunch of villagers in. make sure to have all spawnable blocks covered in slabs. the only place where they should spawn is the water stream.