iPhone XS Camera glass cracking without impact

I have an iPhone XS that was purchased about 16 months ago and kept in mint condition up until now. I always had it in an OtterBox case with a tempered glass screen protector, and the glass back doesn’t even have any scratches. I discovered today that in the past couple of days my rear camera glass has cracked from the outside steel band to the top of the main camera lens. I have never dropped it or had any impact to the camera lens, and there are no scratches or marks on it apart from the crack. Why would this have happened? How much would it cost to fix this defect? I can’t find an option to fix it on the Apple website.