Being billed by two insurances. Help

So my wife had her own insurance in 2025 and it was very good PPO plan good through 12/31/25. She was pregnant at the time and had our son on Dec 16th of 2025. As of January 1st, her and my son would join my work plan. However due to the birth being a QLE, they backdated my work coverage for the family to his date of birth on 12/16/25. Now I am getting EOBs in the mail from my work insurance that wasn’t supposed to kick in until 1/1/26. Her prior PPO insurer has already covered that majority of these bills. What do I do? None of these companies are any help on the phone. For example, one of the bills was $3300 and her PPO plan already covered $3100 according to the hospital billing site. However, the new plan sent me an EOB saying I may owe $2500 for the same bill. Is the hospital just sending the new insurer a request for the remaining $200? If so, do I just hope they deny it and pay the $200 as planned? Or are they both getting billed? I am so lost. Thought I had all of this figured out u til I get the EOB in the mail from the new insurance that wasn’t supposed to start until 1/1/26