The hanging sign is dangling

When you remove the block the hanging sign was attaching, you will get a dangling sign.

What I expected to happen was...:
After removing the block the hanging sign was attaching, this hanging sign should be broken.

What actually happened was...:
Instead, this hanging sign actually became a "dangling" sign, which can be normally used as other blocks.

Steps to Reproduce:
1. Put a hanging sign on any full block.
2. Break the block you have chosen.
3. The hanging sign still exists.