The bug

when a player's falling speed exceeds a certain level, he can no longer get into the scaffolding from the side whether he's pressing shift or not. Since the sides of the scaffolding don't; block the player's movement, that should be considered a bug. (I fell to death in my hardcore world because of that, thinking I'd be able to get back into the scaffolding )

How to reproduce

pillar up from the flat ground with scaffolding for around 100 blocks, then pillar up on top with solid blocks till the build limit. fall against the column while pressing forward. from different heights. You'll not be able to get into the scaffolding if you fall too fast. (shown in the attached video)