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Yes. you don't understand how oxygen sensors work, or how the ECU reacts to what the sensors are telling them.
an oxygen sensor detects unburned oxygen in the exhaust gasses. the more oxygen there, the more fuel the computer decides to pump.
The MAP enhancer circuit may compensate for HHO volume in the intake and adjust adjust your engine timing, but until you have control of fuel delivery -meaning injector pulse duration, meaning correcting the signal the ECU sees from the 02 sensors with an EFIE- you will continue to see a reversal in your fuel economy numbers.
the good news is that if this is (was?) the same 3500 engine as I have in my '07 Buick Rendezvous, it is quite responsive to modifications. IF you're still following here, and if you still have the car, feel free to reach out via DM and I'll share what I've done with mine.