I changed my AFR for e10 from what I was using 14.22 to 14.42. Thought maybe 14.2 was a bit rich and our gas is 'up to 10% eth' according to the stations. So stock was 14.68 on my truck. I figured I would bring it up a notch since my best mileage has been 16.2 and I was getting 16.5 before. Still not great mileage and I need to do a new MAF correction tune. But, maybe I was getting better because of the 'winter' fuel blend. Now summer is here we have 'summer' blend.
So could be that was the difference in my economy going down some?

Anyways, after I reflashed the PE Ratio change and Commanded AFR change, I noticed while I was idleing in neutral on restart, my AFR went very lean (17's). Gave throttle and came back up, but went lean for a minute after let off. Drove fine and AFR was toggleing as usual.
I have a AEM wideband gauge for the record...
PE came on as usual. But when I pulled to a stop and idled, it went lean again. Lean like it stumbles a bit when its going back down to idle speed (800 RPM)
Did it 2 more times after a drive and coming to a stop. Is that normal until it relearns to go so lean when coming to a stop after a drive. After a minute it goes back into normal idle afr weird

To update, I went back to a saved older tune in which I had no changes other than the speedo limiter, upshift light, tire and gear sizes, etc...
The current one as above was PE, AFR Commanded and minor spark alteration. Nothing major tho.
Even with the older tune back on I still have the issue. I can cruise all day long and my AFR stays normal. But when I am slowing to a stop and almost stopped, and my engine is returning to the idle RPM, the afr goes super lean (17's) and you can hear the engine note change...sounds like a slight stumble too.
It finally gave a PO171 and PO174 SES MIL


Anyone got insight?
Is there something specific I can scan for to determine what the cause?
Is there a setting that somehow may have got changed I need to investigate?
Is this some more physical issue like vacc leak, O2 failure, etc? My O2s are abt 10k miles new, truck has 96k miles on it.

Oh its a 2003 S10 4.3 manual for the record. Naturally aspirated