It is possible for a player to open a shulker box when there is a hanging sign in the position above it.
Steps to Reproduce:
1. Place shulker box
2. Place a hanging sign (in any of it's possible states) above the shulker box
3. Press 'Use Item' on the shulker box
Observed Results:
The player is able to open the shulker box
Expected results:
The player would not be able to open the shulker box. This behavior is expected as other similar hanging blocks such as the bell block, dripstone, and lantern all disable the shulker box from opening.
Hanging signs are non-solid except for the bar (if there is one), shulker boxes are only blocked by solid blocks.
Not a bug.