Cannot reset scheduled task to defaults

Since the upgrade to Moodle 2.8 I cannot reset scheduled tasks.
I go to edit the task, check the "reset task schedule to defaults" option, the boxes all grey out and then when I submit it returns me to the form with a "required" error above each of the greyed out boxes.

Despite the fields actually being present in the data array (at least at validation() point):
Array ( [minute] => 16 [hour] => 4 [day] => * [month] => * [dayofweek] => * [disabled] => 0 [resettodefaults] => 1 [task] => tool_langimport\task\update_langpacks_task [action] => edit [submitbutton] => Save changes [sesskey] => blah)