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=falsescaffolding 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
Can confirm in 21w03a.