The illegal Armor Stand bug occurs when multiple Armor Stands are pushed into the same space using pistons . This can cause the Armor Stands to merge, creating an illegal Armor Stand with unsupported properties, such as inconsistent positioning or values. This bug can lead to visual glitches or make the Armor Stand difficult to interact with or destroy properly.
This is a duplicate of MC-196201. The armor stands have the same hitbox, and do not have collision. Them sharing the same block space is not an 'illegal' armor stand.