Looking at the code here:
https://github.com/moodle/moodle/blob/master/question/type/gapselect/edit_form_base.php

Around line 227 One sees the following code:
 

protected function data_preprocessing_choice($question, $answer, $key) {

    // See comment in data_preprocessing_answers.

    unset($this->_form->_defaultValues['choices[$key][choicegroup]']);

    $question->choices[$key]['answer'] = $answer->answer;

    $question->choices[$key]['choicegroup'] = $answer->feedback;

    return $question;

} 

Note the use of single quotes in the line:
unset($this->_form->_defaultValues['choices[$key][choicegroup]']);

This looks wrong because the $key won't be expanded.

Presumably this should either use double quotes or the following form:
unset($this->_form->_defaultValues['choices[' . $key . '][choicegroup]']);