Alright, no changes to the tune and I got this log with timing. No high idle this time.

Dually COS5 Idle Log 2.efi

Remembering after this log that I'd left my ECT fueling modifier all 1.00 (and that my 1500 didn't like that), I stole the values from my 1500 as a starting point for this next log on a separate day and it idled much more happily in closed loop. I also thought maybe a fresh cal flash might get the high idle to come out again but it didn't.

Dually COS5 Idle Log 3.efi

So I guess now I'm on a treasure hunt for a problem I didn't know I had that doesn't seem to want to return.

Quick side question, I was logging GM.dynair for airflow at idle. Is this the correct parameter? Calculated airflow in g/s?

I'll be performing more logs going forward as the weather is cooling off to get the airflow values where they should be and double checking that everything else got moved over correctly so this may pop up again at some point. The truck does live 20 minutes away though.