Scaffold blocks will not allow you to place additional scaffold blocks on either of its sides. Is this working as intended?

oh I think I know what this means finally
you're trying to place a scaffold in midair next to another block
scaffolds need a solid block (or another scaffold) underneath as support, so this is intentional. you can make horizontal bridges with rope

(2 months woo)

Does shift-rightclick not work?

No, shift-rightclick doesn't work.

I'll take a look then