So if your password is "password", it will get entered in as "Password" - and the user will get confused why their username/password aren't logging them in.
So a UX pattern is to actually lowercase the first letter on the backend.
While this technically slightly lowers security (they are trying 4 passwords built from the one you typed in), I don't think that's significant, and I imagine it greatly improves user experience.
You have draw the line somewhere and degrading the majority’s experience for the minority’s benefit is an unusual trade-off.
Whatever happened to, “Design for the expert user”?
I don't understand why this would cause an expert user trouble (it's the loss of a single bit of password security, which shouldn't matter if your password is even reasonably decent).