I have similar fuel trim issues. They seem to crop up after the tune has time to stabilize. Around 30-60 days after running. My AF correction drops to around -35 and af learning drops to around -15. (running rich)
My tuner advised that there is a leak somewhere but I have checked every clamp, replaced the MAF extension cable multiple times, tested the MAF and MAP sensors, and smoke tested this car at two different shops with no issues found. I am running the perrin inlet with nozzle going to a grimmspeed stealth box. My tuner advised that 15+ WRXs inlet is not under vacuum like most cars, so a vac leak actually leads to a rich code. The shops are baffled at this saying that does not makes sense.
Based on this thread, I am guessing the intake and/or inlet need to come off and stock parts need to go back on. Can anyone confirm the inlet is not under vacuum?