The bug
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?).
How to reproduce
Place a scaffolding block two blocks above the surface
Try to jump into it from underneath
→ ❌ You can'tMove 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
Bug report isn't detailed enough in my opinion. The bug title should be "Can't jump into scaffolding from underneath while inside the scaffolding block". The current title makes it seem like you can't jump into scaffolding from below at all which isn't the case. You can jump into it from the side from one block below. The only way you can't is if you're inside the block already.
To me this seems like it is working as intended as the scaffolding is meant to slow the player down when you are walking inside it so it makes sense it would do so vertically as well.