when players open the horse inventory, their own inventory gets shifted 4 blocks to the right, directly under the image of the horse, appearing to delete everything in those slots of their hot bar. opening their inventory after that looks the same, but opening a chest instead will return the inventory to normal. picking up items while in “broken” state will only put the items into the actually available slots, not the ones that appeared under the horse, and trying to put items into those 4 slots
before opening horse inventory (normal):
while opening horse inventory (broken):
after opening horse inventory (broken):
opening chest resets inventory (normal):
hopefully this is an easy fix. thank you for all of your hard work!