If you flex kit is evap connected the rear 02 sensor should be normal. Maybe log the rear 02 sensor volts if you have ecutek cable. It might be wiring fault or short check cable not damaged
Many ecu inputs are pulled high by ecu if open circuit.
also this could be due to running rich as this is high volts on sensor.
https://www.engine-codes.com/p0138_subaru.html
maybe watch here and part 2. unfortunately some tunes may disable ltft so may not work on your tune
[ame]https://www.youtube.com/watch?v=5WnM_NsOtd8[/ame]
check your AFR vs commanded AFR and your long term fuel trims. high trims or afr richer than commanded afr by a large margin may indicate an issue like exhaust leak or a problem with flex fuel system causing car to run rich