My colleague recently bought a phone and also activated gift cards. There were about 10-12 of them (it was an iPhone 13 Pro) and there were no problems at all.
But more to the point, it could easily be that it takes 15 or 20 cards within a short amount of time before the automated systems are triggered. Apple has a lot of data on how scammers operate, and has dealt with scammers many, many, many times, and probably continues to refine their checks periodically. Still assuming you are not a scammer, you are definitely unlucky.