The Monster Spawner won't spawn an Ender Dragon or Wither.

Steps to Reproduce:

1. Acquire a wither spawn egg and an ender dragon spawn egg using commands (Wither: /give @s wither_spawn_egg | Ender Dragon: /give @s ender_dragon_spawn_egg)
2. Place two monster spawners
3. Place a wither egg in one and the ender dragon egg in the other

Observed Results: 

The Wither will not spawn but is spinning inside the spawner. The Ender Dragon "spawns" the model of an Ender Dragon on top of the spawner but it doesn't move. The Ender Dragon is also spinning inside the Spawner.

Important!: The Wither will spawn in the Nether and End but not in the overworld. | The Ender Dragon will spawn in the End but not in the Nether or Overworld.

Expected Results: 

The Wither and Ender Dragon should spawn and move around no matter the dimension.