I found a temporary solution to the bug, the horse may not be visible but it is still there, if the entity is updated (like for example, by taking damage) it will instantly reappear, this can be done via damage potion, lava or suffocation (tp the horse between 2 blocks so that it takes 1 heart of damage)
Edit: I found a better solution!, if your horse dissapears just execute this command so that it takes less than 1 heart of damage:
UltraSoda
I found a temporary solution to the bug, the horse may not be visible but it is still there, if the entity is updated (like for example, by taking damage) it will instantly reappear, this can be done via damage potion, lava or suffocation (tp the horse between 2 blocks so that it takes 1 heart of damage)
Edit: I found a better solution!, if your horse dissapears just execute this command so that it takes less than 1 heart of damage:
/execute @e[type=Horse,name=Insert_name_here] ~ ~ ~ /summon Arrow ~ ~2 ~ {damage:0.1,life:1199,Motion:[0.0,-0.1,0.0]}