My witch farm had a half slab roof over top of it so the witches could spawn during the day, however once I updated to 1.14 the light level on the inside of the farm was too high during the day to allow witches to spawn. The farm works great at night and great if you encase the spawn platforms with all solid blocks. After replacing the entire roof with solid blocks thinking that was the problem the light level on the inside of the farm was still too high to allow witches to spawn. I fixed the problem by removing one random slab and it seemed to have given a light update causing the farm to go back to how dark it was in 1.13.2 however I went back to my witch farm the other day and the light level on the inside of the farm was too high again. You can tell because it seems like its the natural daylight in the farm when it should be completely dark. It seems that slabs now can hold a light level somehow maybe when the chunks unload and load again?