I've succeed to solve it by adding the number 3 (it can be anything else too) here :
question/type/multichoice/edit_multichoice_form.php
around row 70:
$repeated[] = $mform->createElement('editor', 'answer',
$label, array('rows' => 3), $this->editoroptions);
.............
$repeated[] = $mform->createElement('editor', 'feedback',
get_string('feedback', 'question'), array('rows' => 3), $this->editoroptions);