Potion effects with amplifiers above 127 work weird in the new snapshot.
How to replicate:
1. Type in "/effect player_name 1 10 127". For 10 seconds you will be able to run super fast.
2. Type in "/effect player_name 1 10 128". Notice that FOV gets lowered, and you will be unable to move for the duration (you can still move while not on ground).

I checked this with few effects: speed, jump boost, regeneration and hunger. Effects when amplifier is 128:
Speed: I was unable to move.
Jump boost: I was unable to jump.
Regeneration: Instead of regenerating health super fast, it worked as if level 1 effect
Hunger: Same issue as regeneration.

As amplifier is now capped at 255, I think they use bytes for effect amplifier. Vaues above 127 cause byte to owerflow, thus generating potion effects with negative amplifier values.