Before version 1.16, if you were riding a baby horse, donkey, mule, skeleton horse, or zombie horse, you could access all of their slots without any issue. However, since version 1.16, if the mob is turned into a baby, the saddle slot is hidden from the inventory UI. This prevents you from removing or adding a saddle, even though the saddle is still present and functions as expected. It's important to note that this issue doesn't affect the horse armor slot of horses or the carpet slot of llamas. Furthermore, even camels, which didn't exist in version 1.15.2, are impacted by this issue, meaning their saddle slot is also hidden.

Steps to reproduce (for 1.15.2 and newest version)

1. Tame a horse
2. Ride it
3. Saddle it

4. /data merge entity @e[type=minecraft:horse,limit=1,sort=nearest] {Age:-99999}

   In newer versions the following command works too:

   /execute on vehicle run data merge entity @s {Age:-99999}

1. Open inventory

Observed: The saddle slot is no longer visible, but the horse armor slot remains visible.
Expected: The saddle slot should still be visible, as it was before version 1.16.