Did they do that test domestically or internationally?

I suspect it does not matter for international mail because (apparently) /all/ international mail is priority. At least where I am, there is only a service variation for domestic mail. I’ve asked a few different post offices for non-priority Europe-bound stamps. They do not exist. We have a choice between priority stamps and regular for domestic. As the world suckers for the digital transformation and postal usage declines, there are fewer deliveries per week for the lower class mail. It was announced that deliveries have been reduced to like 3 or so times per week for non-priority mail and I expect it to worsen.

Regarding the test you mention, 6 mailings is a tiny sample. A non-priority envelope can perhaps get lucky if the timing is such that it reaches the local post office on a delivery day. Whereas priority /should/ go out for delivery ~5 or 6 days out of the week, IIUC.

I have started using the Event Timer app (in f-droid) and set up a button for priority mail and non-priority. So when any non-priority mail comes, I log it as non-priority. It’s too small of a dataset to be useful but perhaps in a few months I’ll see a pattern.