When you try to place a block inside a marker armorstand, it will initially make the block appear but then the block disappears instantly.
1.
/summon ArmorStand ~ ~ ~ {Marker:1b}
2. Place a block on it's position
Comments 7
You can place them, but they disappear instantly, so you see the block for a split second and then it gets reversed.
Instead of not being placeable at all like with other entities.
I think this is because the client let's you place it but then server doesn't agree with that.
if the armorstand has NoGravity set to 1 then you can't place a block at its location.
testet in 1.8.2-pre7
What command are you using to place the block?