Sudden Drop In Co2 Use After Trimming?

I have had similar thoughts in the past, but in my opinion it is normally a calibration issue. I don`t know how the apex works, I have a GHL system, but I guess they work on a similar principle. On the GHL I can read the calibration data of the probes, so I always note these values before doing a calibration, and then again after. If the values are grossly different, then something either went wrong with the calibration, or the probe is on it`s way out. Once, the probe really was playing up, and the CO2 consumption changed dramatically. Since then I never re-calibrate and make other changes at the same time, otherwise if such an effect like what you`re seeing does occur, then you can`t identify what the cause was.
If you are able to read the calibration data on the Apex (it must be possible), then I would suggest you read this before you re-calibrate next time, and compare the values to those after the calibration. I guess it`s too late for that this time though, since the new calibratin data has already been saved to the Apex.
Also, if the calibration data does change dramatically, it doesn`t necessarily mean you need a new probe immediately. It just means you are operating in a different range, and that you maybe need to keep an eye on it.