I got a notification “You’ve earned the "modular-arithmetic" tag badge”. However, when I clicked on it, I got congratulations for the last tag (card-games), for which I’d already seen the congratulations; whereas to the right of the congratulations, it says “Newest: modular-arithmetic”. Here's a screenshot:
You weren't tracking the modular-arithmetic tag badge there. You were tracking the card-games tag badge, and never clicked the "Track the next one" button to update it to track something else, so it's always just been displaying that congratulations message. You need to click the button to track another badge if you wish to see something else there.
The page is displaying items correctly. Your most recently tracked badge is completed. Your most recently earned badge is not necessarily the one you were tracking most recently - it's possible to earn other tag badges while tracking a different one (such as earning bronze tag badges while tracking progress towards a gold).