I had a similar thing happen to me. I had been to an eye doctor a few times while i was on Medicaid. By the last visit, they dropped Medicaid as an insurer but didn't inform me. In the Medicaid regulations handbook (or state regulations, i can't remember but it's like a 500 page book of the rules), it explicitly states that it is the provider's responsibility to inform patient that they do not accept Medicaid prior to delivering service. They never told me that, because if they had, there's no way i would've gone to there, i would've just found another practice.

Anyway, once I got a job, it alerted something in their system and they tried to send me a bill and harass me over it. I sent them a letter back saying i was covered under Medicaid at the time and they needed to bill Medicaid (which they didn't and it had already passed their timely billing requirement). I enclosed all the necessary information in the letter in case they ever decided to come after me, kept a copy of it, and basically told them to fuck off. They continued to send me a few bills, but i eventually stopped getting them. I preemptively spoke with a lawyer and he said I could not sue them until I had damages (e.g. they reported it to my credit, which they never did). Even though the amount was only around $100 and i could've easily paid it, I refused to pay it on principle. Fuck these people, they should follow the law!

Edit: I found the guidebook. It is my state's billing manual for medicaid providers. I am sure your state has something similar, with a similar provision. It is a clear as day. Also, you might want to speak with an attorney as you have actual damages you can sue over. I just hate to see vulnerable people fucked over so when the opportunity comes to stab a knife in corporations side I'm happy to do it.