For some sites it works well offering your password as you click in the password field - but it’s no better than before.
However, for other websites or apps, I guess those that don’t really want you to save passwords the entire process is worse than before. Previously for these websites I would click in the password field and nothing was offered so I would simply entered the password manually or used the share sheet to open an app such as Lastpass/1Password to inout the password. Now each time I click in the password fiend iOS12 suggests a wrong complex password which I don’t need because I already have a password in Lastpass/1Password so now I have to click “use my own password”, then click the key icon, on the right, then the Lastpass/1Password link to open the password manager app, then click the password from there. It’s about 3-4 clicks more than it was before and if you need to visit these websites several times a days it’s a pain in the ass.
They should have added a “never suggest passwords for this site” option.
I still think the password feature is unnecessarily cumbersome.
I’m still quite confused, sorry. It should only offer to generate a new password for password fields that expect you to enter a
new password, such as those on forms for creating an account or changing your password. In those cases, you won’t already have a password, because the point is that you’re creating a new one, so I’m confused as to why you say you already have it in another password manager...?
If the field is expecting an existing password (eg you’re logging in, or it’s a field on a ‘change password’ form that wants your current password) then it shouldn’t generate a password for you.
Safari uses heuristics and the site’s markup to determine which whether a password field is for an existing or new password and I’ve rarely seen it fail. Perhaps this is what is happening- do you have any example sites?
Finally- and this might be the most important thing for you, as the above may just be a result of me misunderstanding- if you’re using LastPass, you could turn off iCloud Keychain for AutoFill and only use LastPass. This will stop Safari generating new passwords for you. My advice would be to fully commit to one or the other and not use multiple.