You cannot jump into scaffolding from underneath. This is prevented because you are slowed down in it. However you can jump into it from vertically underneath, so that you first reach the required height outside the scaffolding and then enter its collision box.
Expected would be that you can jump into scaffolding from underneath, i.e. you are not slowed down if your feet are not inside the scaffolding (or does that allow overpowered elevators?).
- Place a scaffolding block two blocks above the surface
- Try to jump into it from underneath
→ You can't
- Move right next to the scaffolding, but make sure that your collision box does not intersect with the one of the scaffolding
- Jump up and when your feet are at its height, move forward into it
→ You are standing inside the scaffolding now