Unable to descend scaffolding while standing next to vine or ladder

When stanting on top of scaffolding, and also adjacent to vine or ladder, pressing the "crouch" key will not cause the player to descend the scaffolding, but rather to merely crouch.

To reproduce:

• Build a solid vertical pillar at least 2 blocks high.

• Directly next to the pillar, place a pillar of scaffolding at least 1 block high and lower than the solid pillar.

• Directly above the scaffolding, place vine on the solid pillar.
  (see attached image, 2nd and 3rd set-ups)

• Ascend the scaffolding pillar until you stand on top of it.

• Press the "crouch" key.

Expected behavior:

• The player should descend down the scaffolding pillar.

Observed behavior:

• The player crouches next to the vine.