I can assure you that the phones that Apple gives you as replacement are brand new phones. The only noticeable difference between the two is that in settings, instead of having a model number that starts with M, you will have one that starts with an N. The N signifies that it is a replacement device. Bear in mind however, that these are brand new devices manufactured solely as replacement devices and they don’t come in the fancy retail packaging. My girlfriend has worked in cell phone repair for a couple years and she’s told me that the XS and 11 were known to suffer motherboard damage when being dropped. Now I’m not saying that’s what you have. Depending on how much AppleCare you have left I would suggest you wait a week or so and see. Often times, that stutter you are seeing now is just paranoia and it’ll probably be nothing.