Summary:
Mobs with an odd amount of health show 1 half heart less when riding them (e.g. small slime (does not show health at all), rabbit, tropical fish).

Steps to reproduce:

1. Summon a mob with an odd amount of health (like a rabbit).

2. /ride @s mount @e[type=minecraft:rabbit, limit=1]

3. /data get entity @e[type=minecraft:rabbit, limit=1] Health

   Check how much health a mob has.

Observed results:
When riding a mob with an odd amount of health, it shows half a heart less than the mob actually has.

Expected results:
When riding a mob with an odd amount of health, the mob will show the correct amount of health.

Screenshots: