The bug
Some entities seems to don't have any vertical friction, so they do this curve:
[media]How to reproduce
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"}}
Run:
/summon armor_stand ~ ~ ~ {Motion:[5.0,5.0,0.0]}
Can confirm in 20w51a.