When I was play my MC world I notice my XP bar said I had 0 levels (it was empty) when I should've had 22 levels. I killed a chicken to gain some exp and it set itself back to 22 again. How strange!

This seems to only occur when leaving and rejoining a world riding a horse:

Steps to reproduce:

-ride a horse (tamed)

-save and quit world

-reload world

-dismount horse

-notice how the xp bar reads 0

-kill a mob to gain some xp

-notice that when gaining the xp the xp bar refills to its previous state