Shane,

If you add a fuel source then the immediate response of stft should be go to a negative reading as its taking fuel a way not adding anymore and if you use propane in the air intake you'll see the long term also go negative because its trying to compensate for the rich 02 sensor and the stft reading.

I think a good example of this is a Corsa with an afm issue. You'll see a max reading of + 20% ltft because its adding fuel due to a slow response of the afm which the ecu believes it to be a lean condition and often sets a p0170 error code or an 02 sensor code.

I think you can resrt adaptions or try clearing emission related codes using eobd and then rerun the car to see what fuel trim figures are.

I only have a very basic understanding of this subject but had an issue with a Chevrolet Kalos which taught me a bit about fuel trims and the CAT's ability to store oxygen.