When placing scaffolding under another scaffolding, it will cause the above one to break, even if it is properly supported.

How to reproduce
1. place a scaffold on a block, and another the side of it, with air below it.
2. place a scaffold under the scaffold that has air under it
3. The above scaffold will pop off as an item, even though it is supported on the side.

This is hard to explain, so please see the below video: