Issue:

When zoglins are set to spawn in a biome via datapack, they can and will spawn on air, which is typically an invalid choice for monster category mobs, such as its living counterpart (hoglins).

This behavior occurs in all versions in which zoglins are present; in every version since 1.16. 

Steps to Reproduce:

1. For example, the datapacks attached have zoglins spawn in crimson forests instead of hoglins.
2. Load the datapack into a new or existing world.
3. Find a crimson forest. The issue is most noticeable in large, hollow areas where there is more air/ceiling is higher up.
4. Notice that zoglins will spawn above the ground in air, and sometimes take fall damage upon landing or simply fall into lava oceans. 

Another tip to make the spawns more noticeable is to periodically run /kill @e[type=zoglin] to free up the mob cap.