Sculk veins cannot be placed on the top or sides of soul sand

The bug

Despite soul sand being a full cube, sculk veins can only be placed on the bottom side of the block. Placement on the sides and top will fail.

How to reproduce

1. Place a soul sand block
2. Attempt to place sculk veins on all six sides

Expected results

Sculk veins would be placeable on all six sides.

Actual results

Placement only succeeds on the bottom.