In a datapack, using /ride to make an entity mount another entity makes the first entity's position update 2 ticks later.

Steps to recreate :

1) make entity x (a creeper for exemple) mount entity y (a husk for exemple) with /ride

2) store entity the creeper's y position in a scoreboard and output it in the chat (it has not been updated yet)

3) repeat step 2 one tick later via a schedule command (the creeper's y position has still not been updated)

4) repeat step 2 one more tick later (the creeper's y position has been updated)

An exemple datapack has been provided (run /function ride:start)