I found a discussion on IATN about a 2008 WRX that failed smog and had a P2096 code set. The cause ended up being a displacement of the rear O2 sensor. Looking at the scan data, there was a very subtle difference between the old sensor and the new one. I have attached a photo of the scan data. Look at the data on the left with the old O2 2 voltage sensor and compare it to the data on the right with the new O2 sensor. You will see that the voltage pattern of the old sensor looks like it has shifted down 0.1 volts or more. Like I said, it’s subtle, but it was the cause of the P2096. The tech couldn’t get the old sensor voltage to go much above 0.8 volts even when he artificially enriched the mixture (I suspect he used propane). The new sensor goes over .9 volts under normal driving.
Many people look at rear O2 sensors causing tuning problems because they were taught that rear O2 only tests cat efficiency, which may be true for older cars but not newer cars. And even if the rear O2 hasn’t been bypassed, it’s such a subtle difference between old and new that it’s hard to spot.
I would recommend that you verify that the O2 rear voltage goes above 0.9 volts when the mixture is rich, either artificially or monitor it at full throttle and use a voltmeter to verify that it has not drifted and goes below 0 volts to negative. voltage, most scan tools don’t show negative voltage, but a voltmeter will.
Let us know what you find.