Blizzard has confirmed that it is looking into reports of vanishing Diablo III achievements, but assures players that they are not lost.
The developer wrote on the Battle.net forums:
“As you can probably imagine, our servers are currently really busy handling all the new players coming into the game.”
The achievements server in particular is being hammered, which may be why your achievements are not showing up. It is not that they are deleted, but that there is a (very long) delay in retrieving that data.
We are working on it.”
This is just another problem in a long line of issues that have plagued the game since its launch. Apparently most of the major issues have now been fixed—included the server log-in errors—but peoples achievements appear to still be MIA.
Have you had any issues with achievements?