Hope they do something about iOS iCloud Keychain too. On some websites it is still not possible to save authentication data (auto filling it is another matter, as a first step, I want a 99% solution for saving.)
This is a matter of Safari trying to figure out on a particular web page what is a login form, what is a password update form, and which specific fields correspond to user names and passwords in the form.
Old websites that don't use modern coding standards can make it very difficult for Safari to guess. And that's exactly what it has to do: guess.
A simple example: an old-style website that presents a simple password field on a form. What does Safari do about that? Does Safari know that the web form
requires the user to enter a password between seven and nine characters long, with no punctuation marks at the beginning? Chances are, no. So, Safari will put up its default strong password suggestion. If, after the form has been submitted, the website splats an error in the user's face, that's a problem. The password might have been keychained.
But... if the website has the correct annotations applied to form elements using modern coding styles, Safari doesn't have to guess. And it will get it right.
Likewise, as is the case with the bulletin board software used by this particular site, if the site mixes up password change features with other features like changing e-mail addresses (with its own validation) within the same form, that is a recipe for disaster if you're hoping to automate password saves. So, what do you expect Apple to do? Build a list of every popular website and know what its quirks are? That's not going to happen, I'm afraid.
Time will move on, websites will be updated to HTML 5, and if they're coded properly, Safari will have a fighting chance.