I used to ride a horse like this and it's very annoying!
I then realized that i kept holding a strong contact on the reins even after he stopped and that was one of the reasons he kept pulling down. Release your contact right after he stopped.
If he keeps yanking, put his head up a little bit to make him feel uncomfortable (he will associate uncomfort and pulling his head down without you asking) and also go back to a higher speed and start again the transition.
I hope i could help you :)
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