If scaffolding is placed on a block whose collision box is aligned with the top of its voxel, but is not flat, the scaffolding will not see that block as a lower support and remain a falling block entity.
This does not happen for other gravity-affected blocks (sand, gravel, anvils, concrete powder).

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To reproduce this behavior, place/drop a scaffolding block on top of one of the following blocks with collision:

• Hopper
• Cauldron
• Anvil
• Iron Bar or Glass Pane (same collision box)
• Bell
• Grindstone
• Open Trapdoor
• Door
• Stair (half=bottom)
• Bamboo