In 18w45a the scaffolding blocks could be placed on the sides of other blocks so that the scaffolding block could fall, gravity affected it, as with sand, gravel, etc.

This was ideal for creating a column of scaffolding from above to safely lower large heights.( https://www.reddit.com/r/Minecraft/comments/9wvzdw/i_just_realized_how_useful_scaffolding_is/ )

Now, the scaffolding blocks can only be placed on the sides of other blocks of scaffolding, affecting the gravity when exceeding the horizontal limit. Therefore, it is mandatory to create a previous structure to be able to create the column.