MetLife told me my policy had been canceled before the claim

I bought MetLife pet insurance through work because of an employee discount. When I submitted a claim, I kept getting automated notices saying documents were missing without identifying what was needed. After sending more records and following up, the claim was eventually denied on the basis that the policy had been canceled before the claim date. I say I have written evidence it was active. Customer service later described backdating a cancellation. I am working through it and would love to know whether going through HR helped anyone else.