The team has looked at what’s happening (or not happening) here and why.
If the URL was in the format https://username:password@site/, it would be detected correctly. However, as there is only a single value before the @, the URL is being treated as https://username@site/. URLs in this format, without an obvious password, are generally considered to be “safe” and will not trigger the rule.
I’ve added a ticket to the AppSec team’s backlog to improve support for hard-coded Azure DevOps credentials.