The armor stands in the attached screenshot were summoned with these commads:

summon armor_stand ~-1 ~ ~3 {Passengers:[{id:"minecraft:pillager"}]}
summon armor_stand ~1 ~ ~3 {Passengers:[{id:"minecraft:villager"}]}

I expected the pillager to be standing, same as the villager, but it is sitting. I've tried using the OnGround attribute, but that does not appear to affect display at all. Sitting when a passenger of an illager_beast makes sense, but not on an armor stand.