2014 Ford Mustang 3.7L V6 Bank 2 Lean on startup until engine warms up

I’ve been having an issue lately that has not always been the case. For reference, my car has an aftermarket air intake (AirRaid) and BBK Long-Tube headers w/ O2 sensor extensions, a catted X-pipe, and a Borla Atak Cat-Back. It also is running a tune from MPT. I was running a tune from BAMA before my current tune from MPT and the problem was much more noticeable on the BAMA tune. I say that because it seemed the BAMA tune was just leaner across the board, so it seemed to add to my problem. The MPT tune is much richer and I don’t notice the problem of the engine sounding like it is misfiring, however on cold startups no matter which tune I run Bank 2 is showing way lean compared to Bank 1. Sometimes Bank 1 is running ideal AFRs and Bank 2 is up at 18:1. The problem seems to go away after driving for a bit or just letting the engine warm up. I looked at a datalog that I grabbed with my tune device and I watched Bank 2 AFRs slowly creep toward the values of Bank 1 as the engine warmed up. I’ve been trying to figure out exactly what is causing this problem. Some are saying it is a vacuum leak, though I can’t find or hear one. Some are saying header gaskets, though mine seem fine. Lots of people are saying the pre-cat O2 sensor on bank 2 is bad, but I just replaced all 4 not even 2 months ago. Any ideas folks?