Typically, if you have an air leak in the charge system; I would think it would show as too lean at idle, because the system would be sucking in unmetered air during idle. And then inversely, you would have to rich while under throttle because it would be blowing air out of the leak, thereby losing metered air.
I know a bad EVAP solenoid will throw a too rich at idle code, but typically you would also see an EVAP emissions code as well.
I have been suspicious that whatever is going on has been sensed by the ECM and it has thrown the car into "Limp Mode Lite": Thereby disabling electronic boost command. But when I have observed this behavior; typically max boost has only been about 4-5 PSI at my altitude (5000 feet above sea level). "Limp Mode Lite" disables electronic boost command, which leaves with mechanical boost command only (the first 4-5 PSI at my altitude, which is probably 3-4 at sea level). But every car tends to be different, so maybe yours just pulls more up to the mechanical boost command limit. No matter what, if this was the case I would have assumed you would pull more codes when read, than just the too rich at idle code.
But since you have commented that you are not hearing the howl/whistle on cold start, that strongly suggests a leak in the air charge system somewhere. And it is sucking in unmetered air at the leak point, resulting in less vacuum drawn at the air intake, thus no howl/whistle on cold start. It could be that the ECM is struggling with the discrepancy between MAF and MAP readings, and has been running the car pig rich to be safe (part of "Limp Mode Lite" engine protection protocol), but in that case there should be more codes. Maybe it is such a minor air leak, that the ECM has been gradually adjusting A/F to compensate and it has just reached the point where it knows it is now running to rich at idle.
I can tell you when I developed a split in the end tank of the the OEM IC that it exchanged the normal howl/whistle at cold start for a very pronounced loud whistle emanating from lower in the front of the car. When I stood in front of the car during cold start, it was pretty obvious there was a leak in the IC. But when I had a leaky throttle body hose clamp (the OEM hose clamp is a POS, replace it with a T-clamp): I never did hear the air leak until it got bad enough to blow the hose off the Throttle Body. :lol: No mistaking that sound, pretty much sounds like a gun shot. And I can not remember if the normal cold start howl/whistle sound was less during the period I had the leaky TB clamp. But it may have been.