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Looks something has modified a java class to do something it is not allowed to do? Candidates would be those underlined below (but it might be something else?): Check you have the latest versions then contact the mod authors. If it is none of these, then probably the only way you are going to find the problem mod is by experimenting with removing mods until the problem goes away. Backup your world(s) before removing mods.

First you are telling it to spawn on top of blocks that are not leaves "Heightmap.Types.MOTION_BLOCKING_NO_LEAVES". But I don't think spawning in the air is possible for natural spawns. Even if you wrote your own HeightMap.Types, the code is hardwired to ignore BlockState.isAir(). e.g. see NaturalSpawner.getTopNonCollidingPos() which determines the y location of spawns. In vanilla. flying mobs naturally spawn on the ground. e.g. "slabbing your nether" stops ghast spawns. It's only mob spawners or hardwired things like the dragon fight that can spawn in the air.