Summary:

Placing an Armor Stand acts like a block and removes certain blocks(Tall Grass, Fern, Vine, 1 Thick Snow Layer, 2 high grass and ferns) in an area that Armor Stand takes.

Justification:

You might think that this is WAI because Armor Stands are supposed to be behave like blocks so they will remove these blocks. But here are the reasons why I think that is not the case:

1. You can place blocks listed above again at the Armor Stand's position.
2. Other entities that behave like a block(Item Frame, Painting) do not break any block when placed.
3. Armor Stands can

• Be stacked in 1 place
• Be moved by Pistons
• Ride Minecarts
• Go into the portal
• Activate Pressure Plates
  Which indicates it is a lot like a mob with movement speed 0 than a block.

Demonstration Video