Question about owing money to a health insurance company

My previous health insurance company filled a prescription twice for me after I had already switched to a new insurance company and was no longer covered by them. Months later they realized what they had done and sent me a bill in the mail requesting reimbursement for the total of the medications, just over $13,000. The letter also said if they didn’t receive payment in one month they would send it to a debt collector.

I called them and try and get it resolved, but they said to call my current health insurance company and get them to reimburse me that amount, and then I would pay my old provider. The rep told me she put a hold on the claim while I worked with the other company, and that she wouldn’t send it to a debt collector without letting me know first.

Fast forward several months and I have gone back and forth with the old insurance company, the new company, and the specialty pharmacy for both those companies. I never get a straight answer, they have always tried to pass the bill onto the others, and when they say they’ll get back to me in a few days they never do and I end up having to call back and start from square one.

A few days ago I received a letter from a debt collector requesting the ~13k, so my old insurance company had sent it to a debt collector without telling me like they promised me they wouldn’t do. I am still waiting to hear back from my current provider on if they will reimburse me or not.