Similar to MC-55764 but not considered a duplicate. When mounting a boat, your facing direction shifts to align with the front of the boat. This only alters your horizontal rotation, consistent with expected behavior. However, when mounting a horse, donkey, mule, zombie horse, skeleton horse, llama, or camel, your vertical rotation also changes to match that of the entity, leading to inconsistency.

Steps to reproduce

1. /summon horse ~ ~ ~ {NoAI:1b,Tame:1b,Rotation:[0F,-90F]}

2. Mount the horse

3. /summon minecraft:donkey ~ ~ ~ {NoAI:1b,Tame:1b,Rotation:[0F,-90F]}

4. Mount the donkey

5. /summon minecraft:mule ~ ~ ~ {NoAI:1b,Tame:1b,Rotation:[0F,-90F]}

6. Mount the mule

7. /summon minecraft:zombie_horse ~ ~ ~ {NoAI:1b,Tame:1b,Rotation:[0F,-90F]}

8. Mount the zombie horse

9. /summon minecraft:skeleton_horse ~ ~ ~ {NoAI:1b,Tame:1b,Rotation:[0F,-90F]}

10. Mount the skeleton horse

11. /summon minecraft:llama ~ ~ ~ {NoAI:1b,Tame:1b,Rotation:[0F,-90F]}

12. Mount the llama

13. /summon minecraft:camel ~ ~ ~ {NoAI:1b,Rotation:[0F,-90F]}

14. Mount the camel

15. /summon minecraft:boat ~ ~ ~ {Rotation:[0F,-90F]}

16. Mount the boat

Observed: When mounting the listed rideable mobs, you face straight up, matching the rotation of the mob. However, when mounting a boat, your vertical rotation remains unaltered.
Expected: When mounting the listed rideable mobs, your vertical rotation should remain unchanged.