I've noticed that a vendor's web application triggers Safari's "suggested password" feature at "create password" forms, but does not actually save this password when the form is submitted. That is, after submitting the form and logging back out again, Safari's iCloud Keychain will not include the newly created credentials in the autofill suggestions. (Other credentials from the same domain but different subdomain are presented, so Safari is clearly recognizing the field as a login.)
Similarly, creating a password manually (or logging in with it) is not triggering Safari to remember the password.
It occurs to me that the JavaScript involved in the forms' submissions are obfuscating the action and/or data in some way. What actions are Safari looking for when offering to save passwords?