I've noticed failure to fill the right fields (or any fields) on Lastpass, 1Password, Bitwarden and the KeepassXC browser extension.

What is your mythical "good password manager"?

I'm using 1Password+Firefox+Linux, it fails to find the right username+passwords maybe 10% of the time, mostly because services keep using different domains for login than for signup, so it doesn't recognize it's a valid domain.

In those cases, I carefully review the new domain, make sure it belongs to the right owner, then add it to the list of domains to accept. Now the account list properly show up in the future too, until they again change it. But it gives me a moment to pause and reflect before just moving past it.

I cannot remember any times in the last years where 1Password was 100% unable to fill out the username/password for a website unless the website itself prevented pasting passwords (like my old bank).

But even if it fills the wrong fields, it still provides safety as you wouldn't even see the accounts in the list if you're on the wrong domain, so that's your first warning sign.