We could try to address this by changing WebInputElement::IsPasswordFieldForAutofill() to also return true if the current type is hidden. I don't know whether this would fix it nor whether that would be an advisable change, though.

I tend to think that this would be overfitting.

We would need to make sure that hidden fields are never considered for filling, which could make stealing passwords easier.

If we offer hidden fields for saving, in the presence of a better alternatives (and without automatically popping up the save prompt), it might be useful without too much noise caused.

However, overfitting seems to be a concern to me too -- if this is fits a very rare login-form-handling pattern and we are not sure about the impact on a broader set of sites, perhaps it's not worth it.

Note that the login form on the site does not have this problem. So while the user would need to remember the password from sign-up, they only need to input it once on login.