How short-ranged is the Cancel Blueprint feature?

So I'm currently building defenses for the JavaMatic event. Somwhere within those there is an unfinished blueprint but I can't find it. I would have to tear everything down and I'm actually almost done but my chopping list shows 4 weed stems and the only thing I have used them for recently is that defense building project. I suspect there is a stem floor somewhere within these multi-layered walls that has already ropes added but lacks the weed stems. I have already searched for it by dismantling the walls and stairs and already built floors but I just can't find it. I stood next to those encased MIX.R-Modules trying to get the "Cancel near blueprints" function to work. I tris it from multiple sides and from up above and also checked the blockades. It always says it will cancel 0 buildings.

I have read somewhere that this function has a very short range...but how short is short in this case? and how am I supposed to cancel that blueprint without tearing everything down?