Hi. I'm having the same problem as you (all my passwords without username are flagged as compromised). I even created a thread like yours (you even answered there - Bug in Passwords app: passwords without u… - Apple Community). Now, I have an update: I discovered that I had a password without username that was really compromised (I checked on other platforms; that password appeared in the security section too) and, because I had already deleted that account, I moved that password to the deleted section. As soon as I did that, all the passwords without username that were earlier flagged as compromised were normal again. So, I think this is happening when we already have a password that was really compromised ou easy to guess.
Do you know if, when you updated to iOS 18.4, you already had any password without a username that was truly flagged as reused?