This should only happen if you configure the fuel mix to have a lower value needed to create and burn value. Usually I'd recommend having mixing be perfectly efficient, or be slightly inefficient instead. (12MJ consumed to create 10MJ of fuelmix)
Unfortunately, due to how fuel categories are filtered on the consumer side, I'd have to dynamically add an "unmixed-chemical" category to all fuel consumers, which I couldn't figure out a clean way to implement. This also won't work for fluid mixes.
I might also be able to implement this by making the fuel mixer create a new recipe per consumable fuel, but then you'd need to have a mixer per fuel type. No more mixing coal, raw-coal, crushed-coal, and wood in the same mixer.
