As the title says the modifiedby and timemodified fields of the question table are not set during questions import.
This affect all imports and all Moodle versions since at least 1.9 (I have not looked in previous versions)
This is because of the code

            $question->createdby = $USER->id;

            $question->timecreated = time();

            if (!$question->id = insert_record("question", $question)) {

                error( get_string('cannotinsert','quiz') );

            }

in Moodle 1.9
and

            $question->createdby = $USER->id;

            $question->timecreated = time();

            $question->id = $DB->insert_record('question', $question);

in Moodle 2.0/2.1
this code is in the function importprocess of the question/format.php
lines 395-398 for Moodle 2.1
lines 349-352 for Moodle 2.0
lines 325-330 for Moodle 1.9
I guess this is so minor that nobody ever noticed it. And it doesn't break anything I think !