Mounting a horse underwater while both you and the horse are underwater

What I expected to happen was...:
it would kick me off like if i were to rife it into deep water

What actually happened was...:
it would kick me off like if i were to rife it into deep water. however it acted like i was still on the horse and any blocks i placed immediately disappeared and could not break blocks until i closed and opened my game or remounted the horse when it was on the surface. also works on unsaddled horses

Steps to Reproduce:
1. ride horse into water
2. go under water and wait for horse to sink
3. try to mount horse
4. fly/swim up out of the water after it kicks you off and try to place or break an item