I'm afraid this is a known limitation of the single-sign-on feature in current Centova Cast releases.
To date we haven't changed it because 1) it'd be a fairly big job, 2) it'll be a non-issue in a couple of months when v3.0 arrives, and 3) USUALLY the client will try logging in once before attempting a password reset, and when he does that, it will automatically redirect him to the login form for the actual server on which he's hosted, so the "Forgot password" link will be relative to the server the user is already hosted on and thus it'll work.
v3.0 includes a completely new system for managing multiple servers and will not have this problem.