Very strange custom mob freeze

Summary: Whenever I summon a custom mob using a command block, the game almost immediately freezes. Strangely, I can still break blocks and place them, but not beds, signs or chests. All mob AI stops working, but they remain animated.

What I expected to happen was...
I pressed the button on the command block expecting the mob to appear.

What actually happened was...:
The mob did end up spawning, but the moment it did the bug occurred.

Steps to Reproduce:
1. Use the Mob Summon Command Generator to create a custom mob summon command.
2. Paste the command into a command block and activate it.
3. Game becomes bugged.

Misc: This bug happened to me no matter what version or world I was on, so maybe my game files are corrupt or something like that.