When you try to place blocks where a floating marker armor stand is, only certain blocks like redstone, string, or fences can be placed. All other blocks flash briefly and then disappear.
Steps to Reproduce:
1. Run the commands
/summon ArmorStand ~ ~ ~ {Marker:true,NoGravity:true}
/summon ArmorStand ~ ~0.0001 ~ {Marker:true,NoGravity:true}2.You can place a stone block in the 1st armor stand, but not in the 2nd.
3.You can place redstone in both armor stands.
Intended, if the solid part of blocks being placed is in a entity, it fails to, redstone for example isn't solid, fences don't occupy the full block, so there is a rather big chance you don't place it on the entity.
Marker armorstands still are entities and occupy the space, this is intended