Password manager/autofill doesn't work well enough to be useful

I am constantly running into websites where the password generation/autofill doesn't work the way it should. For example I was prompted to change an easily guessed password but when I go into the website to change the password, Safari does not suggest a new password, it wants to select an existing password as if it is logging in.

On another website I can log in but when I have to enter my password again later to verify a change, Safari does not recognise that the password field wants the existing password and instead suggests a new password. My security is not as good as it should be because in this circumstance I have to use an easy to remember and type password.

If the detection is this inaccurate then there should just always be a generate new password button.

It's incredibly frustrating because I seem to be encountering more of these problems rather than fewer.