I don't know if this is the right spot for this but I fed all six of the cookies to my pet meeralune, just like the news post said, and I haven't received an achievement OR the border. Am I doing something wrong oooor is this a bug?

Have you clicked on the "show required items" dropdown on the Achievement page? That might be the triggering event

That did the trick, thank you!

Came here to say I had the same issue, and while going to the achievements page, clicking the dropdown, and refreshing definitely fixed the issue I think it's safe to say that the achievement not automatically triggering without the extra steps is probably a bug. 😅

Edit: In a similiar-ish vein, it took multiple refreshes of the avatar border page to show that I had unlocked the borders at all, sometimes it would even show all of them locked.

it's not a bug; that's how achievements like that have always worked. in fact, most achievements require some specific action to trigger even if you've already fulfilled the criteria—for example, achievements with a counter like gourmand, bookworm, or any pet stat achievements require you to visit the respective pages displaying those things (food eaten, books read, pet overview page) to update their counter.

earned borders showing as locked is a known bug, but it's pretty much just visual. refreshing a few times should eventually show everything correctly and allow you to select any of your unlocked borders.