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.