The Bug:

Hanging sign GUIs don't always reflect their models accordingly.

This isn't a problem for signs, wall signs, or wall hanging signs, but is a problem for hanging signs and hanging signs with their "attached" block state set to "true". I understand that it may be difficult to understand the issue here so I've attached a comparison analysis image which can be found below that will hopefully help in clearing up any confusion.

MC-256807 - Comparison Analysis.png

Steps to Reproduce:

1. Place down a hanging sign or a hanging sign with its "attached" block state set to "true".
2. Look closely at the texture of the hanging sign within its GUI.
3. Press the "Done" button and look at the model of the given hanging sign.
4. Take note as to whether or not hanging sign GUIs don't always reflect their models accordingly.

Observed Behavior:

Hanging sign GUIs don't always reflect their models accordingly.

Expected Behavior:

Hanging sign GUIs would reflect their models accordingly.