Player's OnGround tag doesn't update when riding a vehicle

When riding a vehicle, the OnGround tag doesn't update.

How to reproduce:

1. Summon a pig and place a saddle on it.
2. While flying in the air, execute the following command:

   /data get entity @s OnGround

   → The tag is set to false (0b).

3. Ride the pig without touching the ground.
4. Execute the command from step 2 again.
   → The tag is set to false (0b).
5. Dismount the pig.
6. While on the ground, execute the command from step 2 again.
   → The tag is set to true (1b).
7. Ride the pig again.
8. Execute the command from step 2 again.
   → The tag is set to true (1b).

Expected result:

The OnGround tag would always be false while riding an entity, like in previous versions.

Observed result:

The OnGround tag keeps its value from before riding an entity, which sometimes makes it inaccurate.