What do I do about a Natera bill that wasn’t pre-authorized?

Sorry for the long post ahead, but maybe someone out there can help me understand what’s going on with my situation, or has had experience with something similar.

I’ve had genetic screenings done through Natera twice. The first time around 2019 was no issue – my EOB matched the bill and I paid. The second time was in 2022, and about 3 months later they contacted me saying they needed my insurance information again. I passed that on to them, and later received an EOB from insurance stating there was no preauthorization and I owed $0. About a year later I received a bill from Natera, much to my surprise. They had information on the back of the bill concerning what to do if your EOB doesn’t match what the bill says you owe. So I tried contacting them via the provided phone number (didn’t connect to anything) and email address (bounced back) so I contacted them through their online form. I sent a copy of the EOB and basically got a canned response that my insurance didn’t cover the costs, so I have to pay the balance. But that wasn’t the case to my understanding. So I spoke to my insurance company and they said that I don’t owe Natera anything since they never preauthorized, and to ignore the bills and let them know if I get turned over to collections. Of course Natera is now threatening to do so, and says that I signed some agreement. I don’t have a copy of any agreement, and it’s been long enough that I don’t recall if I signed anything back in 2022 or not. Now my insurance company says they are asking Natera to send the preauthorization, and if they don’t, I can dispute it.

Now I’m quite confused and don’t understand how they can retroactively preauthorize, and what that means for me? Since I’ve resisted paying, my bill has jumped up another $300 (even though they claimed it wouldn’t be more than $250 total), and the last thing I want is to be turned over to collections. I’m tempted to pay to get it over with, as its been dragging on forever and taking up a lot of my time. Any insights?