So every levitation effect level 128 or above causes this issue. I guess it's because of levitation level 128 and above cause the player to float downwards.
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@Anon Ymus I'm sorry I didn't test this enough due to the fact I was in a hurry. And yes you are right you can't reproduce this with a normal levitation effect. The problem lies in this one: "/effect @a minecraft:levitation 1 250 true" Please have a look again. Also the bug still exists in 15w31c
Unable to reproduce. I suspect you have something in your setup that is messing with your results.