viscid thistle
You can place them and they have collision as intended but keep the green and red boxes which normally would indicate valid and invalid positions when in building mode. Red / green has no consequences regarding placement and they can just be placed next to each other.
Steps to reproduce: Place one. 
Edit: While they have collision for the player, one of the guests walked through them. So they might have no longer collision for NPCs.
Savegame through dm if necessary (sorry, I don't like posting that stuff public).
