Opposing Driver's Insurance Is Demanding I Repay A $41,000 Settlement From Two Years Ago After An "internal Audit". Is This Even Legal?

Location: Maine, USA. Back in late 2022, I was T-boned at a four-way intersection near Bangor by an SUV that ran a solid red light. Both vehicles were totaled, I ended up in the emergency room with broken ribs and severe whiplash, and missed over a month of work. The police report cited the other driver for failure to obey a traffic signal, and two witnesses corroborated the entire thing. The other driver's carrier accepted 100% liability with zero pushback. By the following spring, we finalized a formal bodily injury settlement for roughly $41,500. I signed their comprehensive release of all claims, deposited the check, paid off my medical bills, and moved on thinking the whole mess was settled for good.

Yesterday out of nowhere, I received a certified letter from their corporate subrogation unit. They claim a recent "internal post-claim quality audit" flagged the file and determined their original adjuster made an error regarding comparative fault under Maine law. Because of this desk audit, they now claim the accident was actually 50/50, accusing me of "failure to maintain proper lookout", and are demanding I wire back $20,750 within thirty days or they will file a civil lawsuit against me to recover the difference. There is no new evidence, no dashcam footage, literally nothing besides an auditor hundreds of miles away deciding their own employee overpaid two years ago. I am completely stressing out because I obviously dont have twenty grand sitting in an account.

I pulled the paperwork out of my files and read the signed settlement agreement again. It explicitly states that the payment is a "full, final, and complete release of all claims, known or unknown" and bars either party from pursuing further legal action relating to this incident. Can an insurer actually try to claw back settled funds and breach their own signed release just because they claim their employee made an internal mistake? Should I respond directly citing the release language, or do I need to retain a lawyer right now to shut this down? Any insight would be hugely appreciated.