Jump boost level 15 or higher makes the player take fall damage. I don't know if this is a bug or not, but i found it playing around on beta.
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Cleaning up old tickets: This ticket had been set to 'Awaiting Response', but has not received a response from the reporter (~3 months+) so is being closed as Incomplete. If you feel this is still a valid issue then please comment, or create a new ticket following the Issue Guidelines which includes steps to reproduce the problem.
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According to the wiki, jump boost reduces fall damage by 1/2 heart per level. So Jump Boost 15 should reduce fall damage by 7.5 hearts. Could you provide more information about what height you are falling from and how much damage you take?