Non-"attached" hanging signs cannot be placed underneath walls

The bug

Hanging signs placed underneath wall blocks will always be placed with the "attached" block state set to "true". This is despite the fact that the underside of a straight wall has sufficient space to support a hanging sign with this state set to "false", as can be seen via use of a debug stick.

How to reproduce

1. Place a line of wall blocks in the air
2. Place a hanging sign underneath one of the central wall blocks

Expected results

The hanging sign would be placed in the state with two unconnected chains.

Actual results

It is instead placed with the two chains connected in a Λ-shape.