Mobs do not spawn on soul sand

Hostile mobs do not spawn on soul sand, no matter what the light level is. The difficulty is hard and /gamerule doMobSpawning is set to true.

 

Why is this a problem you might ask? 

1) If you have an area in the nether covered in soul sand that was generated naturally, the nether is basically set to peaceful.

2) mob farms, many mob farms that are not laggy starting in 1.13 and up use half the spawning spaces as soul sand for a shifting floor. This design has now been reduced so much in rates that it is necessary to make a farm so big it creates a ton of lag or use a laggy farm design.