Horses can dismount the player right after they have been tamed

What I expected to happen is:

When I tame a horse, I should stay on the horse.

What actually happened was:

After I tamed the horse, I was dismounted as if I hadn't tamed the horse yet. This does not happen everytime.

How to reproduce (video):

1. Find or spawn a horse.

2. Try to tame a horse.

3. Wait a while while sitting on the horse

4. If you've not been dismounted after some time, repeat.