At the moment the spawn behavior doesn’t work in interface blocks and If I want to spawn a block with one I have to send the X & Y values in a msg to a block on the game level. Simple request hopefully could easily be added :).
Just Use Game World Block Make It Not Solid And Make The Layer Number Higher Than The Rest
Check Out My Game It Uses A Simple Building System Using Things Similar
https://flowlab.io/game/view/1362583
Might Help
I forgot that existed hopefully he adds it soon.