Scaffolding behaves as if it was an air block if the player is not in the same X or Z value as the scaffolding.

When walking along the edge of a block, the player's X and Z coordinates may be past the X and Z coordinates of the block the player is standing on. The issue lies with how scaffolding is coded, where if the player is past the X and/or Z coordinates of the block, scaffolding ceases to function as a ladder, and becomes like if the player was clipped into just a normal block. This also creates the issue of the player falling through scaffolding as if it were through thin air, and taking fall damage, when descending via crouching.

here's a video of it (Exceeded max file size for attachment, sorry!)