Charged $6,000 for two doses of medication – please help

I (27) have been on a medication for severals years which is generally managed by specialty pharmacies. I have not had any issues getting it covered and I only pay $5-15 per dose (I take it twice a month.). My partner got a new job in July, and we had initially planned for me to use one insurance as primary and one as secondary. I ordered two doses of my medication under my old insurance plan (we’ll say Insurance Company and Speciality Pharmacy A) and paid the typical low rate while technically covered by both plans. However, a couple months into my partner’s employment, we were running into significant issues having to deal with both insurance companies concurrently, so we spoke to Insurance Company A and they agreed to retroactively end my policy as of July, so that I was able to fully utilize the superior benefits from the plan from my partner’s employer (from Insurance Company and Speciality Pharmacy B) without any hurdles.

However, a few months later, I received a letter from Insurance Company A saying they were billed for prescriptions after my coverage had ended, and they were requesting to be reimbursed what they paid which they claimed was over $6,000, which I believe is the cost of the medication for those with with no insurance. I have tried multiple times to tell Company A to send the claim to Company B, so that is still pending, but I am concerned Company B is going to deny the claim because even though they were also covering me at that time, they have claimed to only cover the medication if it is ordered through Speciality Pharmacy B; meanwhile, Company A is providing me with the uninsured cost of the medication even though I in fact did have coverage with them at the time I ordered and received it, and it is simply that the plan was retroactively ended after the order was placed. The pharmaceutical company that manufactures the medication has a copay assistance program, but even if they were able to help I know that at most they only offer $3,000 a year without rollover. The whole thing is extremely frustrating because the medication is covered under both plans, but because of the very specific situation with the retroactive plan discontinuation, I am not sure what to do. I feel like the responsibility is really on Company A and I don’t fully understand why they went ahead and paid the uninsured rate for the medication, but they have been a nightmare to deal with and nobody I talk to on the phone understands the nuance of the situation.

Looking for any tips and guidance. I have no issues now ordering the medication through Speciality Pharmacy B, but I am very concerned about this massive bill.

tl;dr, was covered under two different insurance plans (A and B), ordered meds under Insurance A’s speciality pharmacy, then later had Insurance A retroactively discontinued as B had better benefits, but now A is acting like I owe them the uninsured $6,000 cost of the med and B may not cover it since it wasn’t order through their speciality pharmacy