Can dropout break due to a faulty cassette or a bad installation?
It is possible that the rear derailleur low limit screw was misadjusted such that holding the shifter down allowed the RD to get snagged by a spoke. That is a fairly common cause of what happened.
Another possibility, also common, is the hanger (what you are calling the dropout) got bent inward for whatever reason, usually uncareful handling of the bike or crashing, after it left the shop, and the RD was able to get snagged that way
Unless something like a stick or vertebrate got caught in the wheel while riding and was able to reach over and snag the derailleur that way, it's likely one of those two scenarios. If you knew what click the shifter was on when it happened, that would be a clue.
It would be weird if the derailleur was unscathed enough to not need replacing completely. They're unobtainable for shops in a lot of cases right now, so that could be a factor that led to some kind of decision to de-mangle rather than replace it, which is idiotic in normal times but can now legitimately constitute good customer service. If this is what's happened, it could be causing the current bad performance and noises, as they usually wind up too misaligned and sloppy to work great. It's also of course possible they just did a bad job.