Older versions of Questionnaire used to include and default to "Never" in the setting for "Students can view ALL responses".

Later versions are missing "Never" from the option list which means questionnaires default to share "after answering the questionnaire".

There is no easy way to prevent students from seeing others responses and any questionnaires which were set to "Never" and are edited have this setting changed by virtue of the fact the option list defaults to another value.

It seems to be that the option is missing from an array on line 72 of localib.php

The missing element is:

$questionnaireresponseviewers = array ( QUESTIONNAIRE_STUDENTVIEWRESPONSES_NEVER => get_string('responseviewstudentsnever', 'questionnaire'),

with this added back to the array the plugin behaves as it did before suggesting all lang strings and code for conditional behaviour are still in place.