HMO Billing Question

A patient came in a while back for an office visit and a 90732. He has United Health Care through CVMG. I know CVMG has a timely of 90 days as well as UHC. The first entity I billed was UHC. They then forwarded claim to CVMG. Two months passed, I had not heard anything so I re-billed to CVMG. I then received a denial for timely. I submitted proof of timely showing this claim was billed first to UHC. They basically are telling me my proof of timely does not matter because it was not billed to them within the 90 day time frame. I always thought as long as you show proof of billing an insurance within the time frame it was satisfactory. Am I mistaken or should I appeal?

Thank you for your help on this matter.

HMO Billing Question