kT wrote:

From what I've gathered, it's looking more and more like a staging
recontact issue related to first stage engine shutdown behavior,
possibly exacerbated by the near vacuum of the staging environment.

The only other possibility would be complete staging failure related to
the upper stage engine nozzle change out, but that seems unlikely.

I hate staging : http://webpages.charter.net/tsiolkovsky/

I don't hate much, but I hate staging.

Now that I think about this for a bit, what would really be interesting
(for example, real bad) is if the engine burp were related to residual
fuel sloshing as a result of the very slight roll oscillation observed.

It's quite possible the roll oscillation was nominal, but I had a real
bad feeling about that, almost as bad as I had when I saw the foam fall
off the Columbia during launch, and thought is was an SRB plume anomaly.