Yes, I am checking this with a group of people who also bought stamps.
There were some inconsistencies, but mostly it's continous IDs.

However: I am retracting my suspicion that orders might be pre-scanned for now.
Today I got a purchase of 13 unstamped and 25 stamped ones, including a few yellow ones and... yes... one red one.
Super happy right now. 

There is a larger order pending, with a bigger sample size I will report back again then, as it will be easier to see with a high confidence level, if pre-scanning probably occured or not.
However with this relatively small order I just received I definitly got very lucky and find it super unlikely, that pre-scanning occured.