Medication got billed to old insurance for about 2 months after changing jobs and are asking for money for a very expensive prescription, what are my options?

Quick bit of context; I worked at job A with Aetna insurance (health and prescription) until July 2022. I started job B in July 2022, and have Optum RX for prescription insurance. Insurance doesn’t start at job B until September 1, 2022, so I filed under the Georgia State Continuation Law to pay out of pocket for Aetna until the end of August.

So far, so good. I start my new insurance in September, give the new prescription card to my pharmacy, and pick up my prescriptions as usual. That is, until October 17, when I notice my prescriptions have all been filled through Aetna still. I go back to the store, Optum insruance card in hand, and make sure they file under my new prescription. I ask the pharmacist if I’m going to have to pay out of pocket for the scripts filled from Setp 1-Oct 17 under the old insurance, and she dismissed it as a non issue so I didn’t worry about it.

During this time, when the pharmacy finally did change over my prescription insruance from Aetna to Optum, I had to fight with them to get a migraine medication, Nurtec, filled. This is important because Nurtec is a $1,000 out of pocket cost and was filled 4 times between Sept 1 and Oct 17. Optum didn’t approve Nurtec until about 3 weeks ago, January 2023.

Today I get a letter from Aetna, basically saying they now need money for prescriptions filled during those aforementioned dates. I will be contacting them directly, as well as my pharmacy, to try and get them refiled under Optum but my fear is this;

Because my Nurtec wasn’t covered by Optum until January, if I refile the prescriptions from Sept-Oct, that Nurtec (the most expensive medication) will not be covered when I do so. Is this accurate? Beyond just calling and explaining my situation to both insurance providers, is there anything else I can be doing to help my case? I really don’t have 4.5 grand lying around to spare