Summary:
Some blocks (and parts of the blocks) placed inside the selected area in the structure block do not displayed. This only happens if the block is placed near the bottom, north and west side (list of blocks below).

List of blocks:

• Bed
• Lectern (with book)
• Conduit
• Spawner (with mob)
• Piston
• Shulker
• Chest/ender chest
• Enchantmet table
• Head
• Banner
• Item frame (with item)
• Bell
• Sign

Steps to reproduce:

1. Place the structure block.
2. Place the above blocks near the bottom, north and west side inside the selected area.
3. Look at the structure block preview.

Observed results:
Blocks and block parts are not displayed in the structure block preview inside the selected area near the bottom, north and west side.

Expected results:
Blocks and block parts should be displayed in the structure block preview if they are placed inside the selected area.

Screenshots: