Got the same response. After being a Mail Plus subscriber for years, I felt let down when I wasn't eligible for the Pass Plus sub under the same account.
It would save me money because I currently have a (Proton owned) SimpleLogin sub for protecting my email, which I could replace for the cheaper Pass Plus.
I guess you could try using two different Proton accounts, one for Mail and one for Pass. Although I'm not sure if this will create some UX problem/conflict.
I guess I'm one Proton service away from switching to Proton Unlimited. Although the counter-argument in my head is that I don't like companies that try to provide all in one services, like all of Big Tech.