So...maybe I'm just special but my achievements are acting super insane.
There are a few achievements I should have, a few that just won't update at all, and some that it says I have earned that I know for a fact that I haven't.
My achievement issues
Some of my achievements that aren't updating:
Wardrobe - Layers (Ankle: 69 / 25, Holding: 135 / 50, Off my Chest: 516 / 50)
Lumi - Fairy Catcher 83 / 25
Aren't updating but only show up in newest/saved: Saggitarius Fan - 1,882 / 1,500 Sticky - 252 / 25 Assistant Chef - 1,864 / 750 Malerias Little Helper! - 1,726 / 500
That it says I have but I don't: Basic - Commerce - Big Spender 1,682,430,011 / 2,500,000,000
Categories that it says have 0 achievements in them: Basic - Social Pets - Books, Food Wardrobe - Wardrobe, SubQ, Lifelike Dolls Seasonal - Morostide, Fireside, Masq, Vesnali, Survival
I should say that, if I refresh enough times, I can get some of these to show. However there are some that I can't get to show at all.

I've been having the same issues. I just noticed today.
"I like it, I like it, Iike it... LOUD"
I just noticed that an achievement I got yesterday, Fast as Lightning, is not showing up as completed on my achievement page.
I think the wardrobe layers achievements have been broken for a while, but I hadn't heard about any other issues until now.
❄ [tot=iciness] ❄
When I viewed my achievements for Fireside yesterday, the Charitable one was at 1064/1000 from previous years. I had to donate an extra item to get the achievement to register and get the book. Today only the 1.000 shows. I hope the extra one register should even more tiers be added in the future!
...actually there should be a few hundred more, because I donated more than the one needed to activate in order to get some of the new books...
All in all I believe LOTS of people still suffer from the achievement transition 2 years (?) ago, missing a bunch of achievements. I myself did not have the patience to check all of mine and now the old achievement page cannot even be viewed any more so we could compare the was to the is state.
I too have experienced the random giving or not giving of achievements. Some achievements that I just earned won't show up unless I "save" them (using the check marking button) to the save/watch list. Only then will it give me credit for having received achievements. I had to go back and do that to many.
I just noticed I had Bulging at the Seams completed, but not given credit...so I checked the box by the title, and refreshed the page. It gave me the achievement. This may be a temporary way to "fix" the issue...but it still exists as a problem.
Also of great frustration is my inability to see my friends achievements accurately. I'll go to their page and look to see if they've received "Free all the Fairies" achievement or another similar item-based achievement. I want to help them obtain their goal, but instead of seeing their achievement, the screen will 90% of the time display MY achievements instead. This happens whenever I click anything on the friend's achievement categories. Sometimes I can view the achivement if I know the exact name and put it in the search field but I get one chance to see it, and never can search for any others on that particular screen, because it reverts to my achievements instead.
As you can imagine, it makes it nearly impossible to see if people need egged, tp'd, pumpkins, fairies, you name it....unless I specifically ask them how many more of (whatever it is) they need.