Holding a block for example and right clicking a hanging sign, does not edit it, but tries to place blocks, regardless of pressing shift.

For normal signs/wall signs, this works when not holding shift, as expected.

The reason for this is probably described in this image:

The hanging sign allows 2 different placement styles, which would not be possible anymore (easily) when it would instead edit the top sign. This could be overcome by adding a check for right clicking the bottom of the sign, vs, the front and backface.