I have just had a similar problem with my S20+ 5G. It was resting on a wireless charging pad, which was connected to the supplied charger and afterwards the back glass cover was cracked.
The phone has never been dropped, shocked or damaged, and has been inside a protective cover since day 1. If you google for photos of a disassembled phone and compare it to the damage it lines up pretty well with the location of the battery. The back is also smooth with no cracks making it to the outer surface, and I would have thought that any external force would have created damage on the outside of the phone.
I'm in the Netherlands, so my phone is currently with Dynafix and their entire diagnosis is 'we see a crack, so it's obviously accidental' whilst paying no attention to the description of the issue that I gave. My only means to communicate with them is a link to a minimal page with 3 buttons - pay for the repair, pay to have the phone returned, or let us dispose of it for free, so I'm not even given an opportunity to question anything.
Where do I stand? I'm concerned that even if I did give in and pay for the repair then there's likely an underlying problem which may reoccur and could even do the same damage a second time.
