Mob trigger places mobs 1 block higher if void_air or cave_air is present, but not air.
What I believe is happening is that MythicDungeons check if a block is present (except for air) at the trigger, and if so places the mob 1 block higher so it goes above that block, but in some cases like using the void_air or cave_air blocks (maybe even structure void & other passthrough blocks, but untested) it makes the mob spawns 1 block higher when it'd spawn the mob on the block just fine.
MD Version 5.6.3 MC Version 1.19.4