Jump boost 255 has one useful property - it completely negates fall damage without affecting jump height (which is very useful for map makers).
But if give myself this buff, then return to the main menu and then load my world again, it no longer works: I still see buff indicator in my inventory, but the character takes full fall damage. Also, I see no usual green particles around my character.
Reconnecting to a server has same effect.
How to reproduce:
Create new world with cheats enabled
/effect @p minecraft:jump_boost 1000000 255
/tp @p ~ ~100 ~
You take no fall damage
Go back to the main menu, then open your world again
Notice the lack of green particles
/tp @p ~ ~100 ~
Boom, you're dead. Fall damage is no longer negated.
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Skylinerw
While the use of high amplifiers is not supported, I'll explain why this happens:
The Amplifier tag is a byte, with a range of -128 to 127. When you use the /effect command, you are setting the internal amplifier, which is an integer (which has a much higher range).
When the internal amplifier is saved, it will overflow to be within the byte range. 255 becomes -1. This occurs when you exit the game with any effect with an amplifier outside the byte range. Negative amplifiers also will not show particles.
If you're working on a map and need to rely on high amplifiers, you'll need to constantly set the effect.
Unsupported amplifiers.