Summary:
During the execution of the /fill command and filling the area with scaffolding blocks, some of the scaffolding may simply fall (not dropped). This happens when you fill the allowable area for scaffolding and you can manually place the scaffolding in the same place later. Also works only with extreme west-north blocks.

The same thing happens when placing scaffolds using a structure block. However, if you set the structure to rotate (in the structure block) during placement, the sides may not be west-north. Also, when placing the scaffold using the structure block, the extreme blocks do not fall, but break and fall out in the form of items.

Discovered while building part of the map for testing MCPE-120896.

Steps to reproduce (1):

1. Stand on a full block.
2. Run the command: /fill ~ ~ ~ ~ ~5 ~ scaffolding
3. Run the command: /fill ~-6 ~5 ~ ~6 ~5 ~ scaffolding
4. Run the command: /fill ~ ~5 ~-6 ~ ~5 ~6 scaffolding

Steps to reproduce (2):

1. Build the structure as in the picture (1 of two).
   
2. Save the constructed structure in the structure block.
3. Place the previously saved structure using the structure block.

Observed results:
The extreme west-north 2-3 blocks will fall/break.

Expected results:
No scaffolding blocks should fall/break off when filled if they have a support and no more than 6 blocks from the support.

Image: