I follow your math and don’t disagree with it - just basing my answer on practical reality.
I have a copy of a several payoff docs from my last BMW lease that I pulled at different dates. BMW shows the remaining payment balance and then “unearned amount” which is interest portion of remaining payments. The unearned interest per month (unearned interest / number of months remaining) was exactly the same amount regardless of when during the lease I pulled the payoff.
Ultimately happy to let you “win” this one but not sure your mathematical theory follows reality.