To be honest, my strong inclination is that something I did wrong with the case cleaning and prep caused this somehow, and not the powder. I just want to consider all angles. Like I said, the powder looks and smells fine, and there's no sign of any corrosion of any sort on either the pulled bullet bases or the brass.
I'm going to do some more tinkering. I have a dozen uncracked, annealed, pulldown cases that I'll reassemble and test, as well as unloaded brass prepped the same way. I'll load and chronograph them. I'll even load some control rounds using powder from the unopened keg to look for any velocity discrepancies.