Keychain Autofill not working on a specific safari site

You may be in luck because I believe the website code has to explicitly exclude Autofill from working, so the web team may have inadvertently done that.

If they are amenable to guidance from us lowly users, point them in this direction:

https://developer.apple.com/documentation/security/password_autofill/enabling_password_autofill_on_an_html_input_element

Be sure to point out the fact the Safari Password Autofill feature ≠ autocomplete despite the fact even Apple seems to conflate the two.

Some websites are annoyingly unhelpful and will and actively block Safari Autofill, "for your security" of course. Practices such as you describe usually result in the end user writing those passwords down on a Post-it Note® affixed to their computer—a great idea that conveniently transfers responsibility for password security away from them.

You can complain to the website administrator, which is almost certain to be a waste of time because they will claim their approach to security is far superior to that of the most valuable and security-conscious company on Earth.

I don't know of any workarounds other than to try a different browser for those specific websites.

Aha. Good work on your part!