Basically, when falling from a significant height while riding a horse, neither you nor the horse will take fall damage. The fall damage will, however, be saved by the horse, and when you dismount the horse, the horse takes all of that fall damage at once. (Instantly killed my horse.)
PS: Also saw that there was a weird bug when going up blocks on horses, but I already saw that reported.
Thank you for your report!
We're actually already tracking this issue at MC-147897, so we will resolve and link this ticket as a duplicate.
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