Unlike regular armorstands, armorstands with the Marker:1b tag will have their held items and equipment blacked out due to a lack of lighting when inside a block. If they don't have the invisible tag, they will also have their body blacked out.
Attachments
Comments 3
This is normal. Entities are dark when inside blocks like that. The difference between your two armor stands is the hitbox. Turn on hitboxes and compare, you'll see how one of them is inside the block and the other isn't.
I think this is intended. See mob in block.