Update from me after more testing, furnaces with fuel that's also a spoiled result all seem to be fixed, but it appears that was indeed a different bug than the mixed fuel one in Py's Coal Powerplant. This mixed fuel bug has nothing to do with the spoilage mechanic from what I can tell. It seems it'...

I'm getting some similar issues, may not be entirely relevant but may also shed some light on what's happening. I'm also in Py but I've been able to reproduce my issue in base game + Space Age. My original issue, in Py, was that my copper smelting stack occasionally had a piece of coke on it's outpu...