The bug

Armor stands can't be placed if there is another armor stand obstructing it – even if it contains the marker tag.

How to reproduce

Summon an armor stand with the marker tag (/summon minecraft:armor_stand ~ ~1 ~ {Marker:1,NoGravity:1}) and try to place an armor stand on the block where the marker is or below it. Unless the marker armor stand was perfectly centered which can be done by using absolute coordinates when summoning it, the armor stand does not place.