When a new privilege is earned, one can see the following card on their profile page:
Now, if I understand correctly, the progress bar's value indicates either the progress of the newly earned tag (which should be 100%) or (unlikely) the overall progress of all tags. If it's the former, the displayed value is (obviously) far less than 100%. And if it's the latter, I checked on a different site with a different privilege and the displayed value was identical:
Also, checking the HTML element shows that it should be at 100%:
But the CSS width
property is set to 0%
(thanks to John Omielan's answer below) and that seems to be the cause of the issue.