The bug

Some entities seems to don't have any vertical friction, so they do this curve: 2018-06-10_10.16.56.png

How to reproduce

1. Run in an always active, repeating command block:

   /execute at @e[type=armor_stand] run summon falling_block ~ ~ ~ {Time:1,BlockState:{Name:"minecraft:lapis_block"}}
   

2. Run:

   /summon armor_stand ~ ~ ~ {Motion:[5.0,5.0,0.0]}