Quite simply, I had a xp trap working before the new snapshot, and now it doesn't, the spawner won't spawn any mobs. I'll have it pitch black, stick around for a few minutes, etc. I went back to the old snapshot and it worked just fine.

Put the summary of the bug you're having here

What I expected to happen was...:
Mob spawner to spawn skeletons in pitch black

What actually happened was...:
No skeletons spawned over a repeated period of time

Steps to Reproduce:
1. Legit, just find a spawner, won't work, blamo
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