the bug: if you summon a horse with the no ai tag and you ride it it it moves on its own and if you get off and get on again it flys up ascend then descends 

How to trigger the bug:

step one: summon a horse with the no ai tag 

step tow: get on the horse it should start moving on its own 

step three: get off the horse

step four: punch the horse 

step five: get back on the horse 

 command i used: /summon minecraft:horse ~ ~ ~ {NoAI:true}

also if you get off the horse while it is in the air it stays in the air without going down