Piglins and Hoglins do not spawn on nether wart blocks, on which other mobs can spawn. They spawn regurarly on warped wart blocks and other blocks.

This happens in both nether wastes and crimson forest biomes but it is more noticiable in crimson forests.

To replicate this bug go in a nether wastes or crimson forest biome and create a platform of nether wart blocks above bedrock to despawn all other mobs below. Observe no piglin and hoglin spawn.