Custom Potion Effect Ampilfier doesn't work correctly

Using a custom potion with a high amplifier e.g. 255, the amplifier is lost when the potion is drunk. E.g.

\give @p minecraft:potion 1 0 {CustomPotionEffects:[{Id:25,Amplifier:255,Duration:20}]}

Gives a potion which gives levitation 255 (hovering). However upon consuming the potion, the level 1 (amplifier 0) effect is applied which raises the player instead of holding them steady.