I attempted to use scaffolding as a backdrop for a waterfall, to then place buckets of lava in and create a nether portal.
I expected the lava to be placed on the facing side of the scaffolding block.
Instead, the lava disappeared.

When right clicking on scaffolding (crouched or standing) with a water bucket, the water fills that scaffolding block with water.
However, when right clicking on scaffolding (crouched or standing) with a lava bucket, the lava is not placed on the side of the block, but rather disappears leaving you with an empty bucket.
If the scaffolding block has water in it, right clicking with a lava bucket (crouched or standing) will produce the same effect.
It does not seem to matter whether or not the player is standing or crouching; if the scaffolding is on the ground, 1+ blocks high, or an offshoot hovering; the scaffolding block is waterlogged or not.

To reproduce:

1. Place a scaffolding block (minecraft:scaffolding)
2. Right click on the scaffolding block with a lava bucket (minecraft:lava_bucket)