Monster spawners are not showing mobs or entities after middle clicking the spawner with CRTL + Middle Click and placing on the ground the mob/entity will not render and spawner will appear empty like so in these screenshots.

Steps to reproduce: 

1. You need to create a mob spawner with a mob by placing the spawner on the ground and placing a spawn egg in it.

2. You need next CTRL + MIDDLE CLICK the spawner to get a spawner of that mob.

3. Then finally you need to place the Mob spawner block.

As you can see the Spawner on the right-hand side does not display the mob of a ghast mob.

Thanks for taking a look into this and have a nice day.