When an armour stand has the "Marker" NBT tag, it cannot move in any way other than teleportation. It's probably intended that it doesn't react to water etc., but not even directly setting the "Motion" tag works.

To reproduce:

/summon armor_stand ~ ~ ~ {Marker:1,NoGravity:0,NoAI:0,Motion:[0.0,10.0,0.0]}

Result: The armour stand stays in place.

The same applies to a few other things, like paintings, AEC, etc., but it might be intended for those.

^{(Putting the American spelling here as well to help with searches: armor stand)}