The Bug:

Hanging roots cannot be placed on honey blocks.

Steps to Reproduce:

• Attempt to place a spore blossom on the underside of a honey block.
  →  Notice how you are able to do so.
• Attempt to place some hanging roots on the underside of a honey block.
  →  Notice how you are unable to do so.

Information:

Since spore blossoms can be supported by honey blocks, I'd expect the same behavior for hanging roots.