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.