If you place a scaffolding block on top of a ladder, the space between the two blocks will restrict you from moving upwards and you cannot pass through. This "barrier" applies only for the bottom.
MC-175823
Scaffolding placed on top of ladders won't allow you to climb the scaffolding properly
Resolved
Cannot Reproduce
0
The bugs it that the player can’t get through floating scaffolding anyways, unless the player has levitation. So if you get pushed by a slime block you can’t get through, while mobs can without any issue.