Check out this forum: https://www.loansafe.org/forum/forums/stop-foreclosure-and-tell-us-your-story.4/
They were able to help me a few years ago when we were underwater on our mortgage. A lawyer started the forum when mortgage companies were not being helpful. Although you are not having upside-down issues, they may be able to offer helpful advice.
D1 went through something very similar, when she turned in her lease car and got another. The rep went on vacation that evening, before ensuring his paperwork was properly handled.
Long story, but we eventually learned this sort of error requires a lot of folks on both sides (old account, new account, old turn-in finance reps, dealer, etc.) The dealer could prove the final payment was processed, the lender couldn’t see it.
Good luck.