The bug

Summoning a marker armor stand with other entity passengers has the passengers offset upwards, even though they should be aligned to the marker stand's position with no offset due to there being no hitbox size.
This did not happen before 16w snapshots.

How to reproduce

 /summon armor_stand ~ ~ ~ {Marker:1b,Passengers:[{id:armor_stand,Marker:0b}]}

 /summon armor_stand ~ ~ ~ {Marker:1b,Passengers:[{id:cow}]}

Some things do work however, such as a marker armor stand passenger or a villager passenger.

See screenshot of pig and cow and armor stand for a picture of the issue in 1.9 pre-4