Got an achievement, now it's missing, and cannot be unlocked again

I have been playing assassins creed rogue, and yesterday I unlocked the achievement "killing machine" for "killing 30 guards with only the ENDURANCE cheat active"

I return back later and the achievement is missing. I try to unlock it again, since it's fairly simple, and now it won't unlock. I unplug and reset everything, delete the profile, clear cache, redownload my profile and still nothing. I online chat with support
yesterday, and they say yes it's unlocked, just wait and it will show up eventually. I check today, try all the steps again today, and still nothing. Contact support chat again, and they say it's now locked again. So I go into the game and attempt to unlock
it again and it still won't unlock. This is very frustrating and irritating because this is one game I can get every achievement for. So now support says post here, so here I am.

:)

 

Gamer pictures missing

I recently moved my account to a new console and all unlocked achievement pictures are now missing, how do I get these unlocked pictures back ? I.E I have got seriously one achievement on gears of war, I unlocked the picture you get but now it's gone? How
do I get it back ?

 

Gamerscore nearly back to normal - but unlocked achievements not adding to score

I've also got the two unlocked-but-missing achievements now. This is now a non-issue

 

I just went into the game and checked the statistics, and it shows that I've completed killing 30 guards with the endurance cheat on. Have pics to prove it, if only I could post them here

 

Here's the in game statistics showing I competed the achievement

http://i.imgur.com/cTpC0op.jpg

Here's my achievements with it missing

http://i.imgur.com/ghGirF1.jpg

 

The achievement is for killing 30 guards without dying. If you died then you would not earn the achievement. Please try the steps at the link below to earn the achievement.

www.xboxachievements.com/.../92716-Killing-machine.html

 

Yes I know it's for 30 without dying. Like I said, I unlocked the achievement. I saw it. Then I came back later and it was gone. I then went out and killed 50+ without dying, all guards, and still nothing, because the game already thinks I unlocked it.