Website have hidden form fields which capture user id and password. If you have autofill enabled without prompting, malicious websites may be able to steal your stored credentials. The pop up comes up when the form is loaded with those hidden fields and the password prompt shows up because we disable autofill by default. This is a security feature that is implemented in Brave to never autofill password.
There is nothing broken here its the website that has implemented the hidden id/password fields which Brave is stopping from autofilling it. This is more of a useful feature than problem. Hope this clears your doubt.