Timely filing denial question

In September 2021, I switched OBGYN offices at 23 weeks pregnant because I did not feel I was receiving adequate care. At my new office, I signed a release for a records request from the old OBGYN office and finished out my pregnancy with the new office. I never made another appointment with the old office and I was never contacted by them again until March 2023 when I received a bill in the mail from my former OBGYN for the delivery of my child to the tune of $13,000+ with my daughter’s birthday attached to it. Very confused and concerned, I contacted the old office and was advised that it was a mistake and the office was “never alerted” that I switched offices and that it would be fixed.

October 2023 I received a bill in the mail for $1,650 for 7+ antepartum services. I once again contacted the office trying to understand this bill. After a LOT of back and forth over the next 4 weeks, I was told by the billing specialist that because the office was “never alerted” that I switched care, they were unable to file my insurance correctly and had to go back in in March and re-bill everything. Because it had been 2 years since the dates of service, my insurance denied it for timely filing, and now I am expected to foot the bill, even though they have been given proof by my new OBGYN that a records request was in fact sent, and they are still claiming they never received it or sent my records. I asked them to send me all of my itemized bills and EOB’s. In the EOB that was kicked for timely filing, it shows the denial reasoning, however it also says “Patient responsibility $1,650.” I spoke to the insurance company (Anthem) who looked over the EOB and advised that it is against the offices’ contract with Anthem to ask me to pay that bill, and it’s on the office for not filing within the 6 months after the dates of service. I asked if there was anything I could send the office for proof that it is against their policy but did not get an answer. I am afraid the office will try to fight me on this even more because it says “Patient responsibility.” Does anyone have any insight on this and how I should proceed?