Insurance Overlaps / Denial of Pre/approved Claims

This is a complicated situation. I notified my employer I was resigning and to cancel my health insurance (United Healthcare) effective 03/31/22. I started a new position and my Aetna insurance coverage started on 04/01/22. My Aetna insurance was my only insurance from 04/01/22 – 04/31/23. My past employer confirmed they cancelled my policy effective 03/31/22, but somehow an automatic renewal occurred and this insurance was extended until 12/31/22. I was not paying for this insurance and Aetna was my primary and only coverage to my knowledge. I now am receiving denied claims from Aetna (after they were previously approved) because my claims were sent to United Healthcare. I am trying to resolve this with UHC, their oversight insurance company, Advanced Insurance Services, Aetna, and all my medical providers. When I make my telephone calls I document everything. Has anyone else experienced anything even remotely like this and how did you resolve? Are there any agencies who assist with circumstances such as this? Any insight and suggestions are very much welcomed. Thank you.