I'm afraid I don't understand what you are doing exactly. Can you provide a minimal working source file that shows the problem?

As you can see in this example, the results are annotated per question (in All_students.pdf), but they are missing the grade in the left upper corner (as with other tests).
However, I wasn't able to find how to turn this feature on for the annonymous exams.

You must use the \AMCformBegin command at the beginning of the answer sheet:

\onecopy{\ncopies}{
        % question header
  \small
  \insertgroup{geographie}

  \cleardoublepage
  \AMCformBegin
        % form header
  \AMCform
}%end \onecopy