The bug

It is not possible to place a scaffolding with the bottom property set to a value other than what would be supported at that location, i.e.:

• scaffolding on ground: bottom=false
• scaffolding in air: bottom=true

This is different to nearly all other blocks where it is possible to place non-default block states.

Reproduction steps

/setblock

While standing on the ground, use:

/setblock ~ ~ ~ scaffolding[bottom=true]

The scaffolding instantly changes to bottom=false

BlockStateTag

Right click with the following item on the ground

/give @s scaffolding{BlockStateTag:{bottom:"true"}}

The scaffolding instantly changes to bottom=false

Debug stick

Try to change the bottom property using the Debug Stick
The scaffolding instantly reverts back to its previous state