I was in a survival in the new snapshot and I was looking for one of the new biomes. I was on the bottom of the nether at the time. I looked up and found the crimson forest. I had scaffolding with me, so I was going to build my way up there.

The problem was, I couldn't place scaffolding on soul sand. I went and checked my controls, and they were fine. I tried looking up the issue and found nothing about it. I went into creative to test it out:

Scaffolding can not be placed on soul sand at any angle.