Password Update Failed

I needed to change my password to a site today, and when I did, I allowed the Safari "use strong password", which is supposed then be remembered by Safari's AutoFill, and saved in my Keychain, on my iMac & in the cloud. Well, the site accepted it, said my password's been updated, but then when I tried to sign in, Safari put my OLD password in, as did my Keychain! So I have no record of the password anywhere (since the window showing the new "strong password" doesn't allow me to copy it, as it doesn't even show me the entire password (which makes zero sense).. So now what do I do? Why didn't the new password end up in my Keychain or in the Safari list of passwords?