After extensively searching I'm really surprised to see that no one has reported this recently, or maybe I overlooked the main thread. One main thread was resolved as fixed a while ago ( https://bugs.mojang.com/browse/MC-13662 ), but this problem is still happening.

Steps to Reproduce:

Mount a horse/donkey/mule, pig, or llama in survival mode. Compare the locations of their health bars and the missing hunger and/or XP bars to when the player is dismounted and walking around freely.

Expectation: Hunger bar and XP bar visible on these mounts. XP bar right above the jump bar for horses/mules/donkeys, and above these two bars would be the player's hearts on the left and the mount's hearts on the right. horses'/mules'/donkeys', llamas', and pigs' hearts shown right above the player's hunger bar on the right.

Actual: Only player's hearts and mount's hearts are displayed without the hunger bar, and on horses/mules/donkeys, the jump bar replaces the XP bar.