Achievements Dissapeared...

All of the achievements you got in the original game were unlocked offline. That may be the problem. If you earn achievements offline you must be signed in to that copy of the profile next time you sign in.  There are 2 copies of your profile - offline and online. They are usually always in sync. But if you take a copy to a mates place or have an offline console they can get out of sync. If you then recover your profile you replace your offline profile with the out of date online profile.

OP I've had a similar strange occurrence.  A long time ago I beat RE's Desperate Escape, and unlocked multiple achievements for hat particular DLC.   I recently went back after about year to earn some more achievements and saw that many of them were gone.  I know for a fact I beat it and earned the achievo, even though it said I hadn't.  Not sure whether it was caused by me deleting it, deleting my Xbox's cache, or a simple update.  All I know is it shouldn't really surprise us that much.  Twice now I signed into LFD after not playing it for months and immediately earned two achievements.  Both of them are multiplayer achievements, which I never play.  Good stuff right?  So what ever happened with your DS2 problems?  Seems like you've been having some crappy problems with multiple games.  Bummer.

@ SatNight: It's been about a year since I played the original Batman AA version... I don't remember anything out of the ordinary back then, like if I wasn't connected to Live or whatever, but I do not believe I have ever recovered my profile at a friend's house ever nor had to retrieve it on my own console for whatever reason. But some sort of connection problem may have been key. That sounds most logical.

@Thookins: Yeah, I have been having lots of weird miscellaneous problems lately, Thookins. Thanks for the time for an extended reply in both threads btw. As for DS2, I haven't encountered any problems since the update on 07/18. Might have been the update, might have been that I am now on Disc 2, might be that I have saved over another empty slot, who knows...

lol, I should pop in L4D just to see if I get a freebie.

IS try recovering your GT.

if you unlock achievements offline and then do a account recover w/o signing in online, you will lose ALL achievement  you gained during the recovery prior to signing in online

That doesn't work in my case.  I haven't had to recover my account in a few years.  All of my issues happened within a year.  It certainly doesn't explain why I've received two free achievements in LFD upon immediately reaching that game's menu for that first time in a few months.  

More people on those forums have said that they've got those same achievements for doing nothing.  Just plain weird.