Non-attached signs are not rotating in 12 directions, even though they could.

What I expected to happen was...:
Hanging signs placed on the bottom side of a block should be able to rotate in 12 directions. 

What actually happened was...:
Hanging signs with "attached: false" only rotates in one of 4 directions, and cannot be placed diagonally. Rotating the sign with the debug stick revealed that there are 12 possible directions, and the F3 screen shows that the 4 naturally possible rotations are 0, 4, 8, 12.

Steps to Reproduce:
1. Place a hanging sign in the bottom side of a block, while looking at it from different directions. (results in one of 4 directions)
2. Change the sign's rotation value using a debug stick. (results in one of 12 directions)