I can't recall ever getting a bag swab when opting out. The whole point of the "patdown" is getting a full body rubdown for a swab test, so it seems like it would be totally redundant.

The only time I can recall an automatic bag swab was when I forgot my ID, so I was flying as a 'selectee' and got the full royal treatment.
I wonder if it has anything to do with your passport, maybe traveling on a foreign passport + opt out is a special combo?