Random Spawner not spawning in Nether

I am using a random spawner to spawn a mythic mob in the Nether. Using "Action: REPLACE" works as intended, but "Action: ADD" does nothing. ADD does work in other worlds, just not in the Nether.

Random Spawner: https://pastebin.com/9d0FjyFD
Mythic Mob: https://pastebin.com/Vc4aDwzj
Config: https://pastebin.com/x9j9wSmG

Using MythicMobs 4.0.1 with Spigot 1.11.2.