In Moodle 3.1 tools can be published with "Publish as LTI tools".

The grades are synchronized through the moodle cron task on the provider to the consumer system. This is only done if the grade is different (that is has a different value then the lastgrade field of the enrol_lti_users table). The grade is however initialized on 0, which means that grades, which are 0 are not synchronized.

Impact: the impact is that it is impossible to see whether people got a grade 0 or did not do the task at all.

The offending line seems to be $userlog->lastgrade = 0; (line 185 in https://github.com/moodle/moodle/blob/MOODLE_31_STABLE/enrol/lti/tool.php). This is still the case in 3.1.3.

In enrol/lti/classes/task/sync_grades.php the values are compared in the line if ($grade == $ltiuser->lastgrade) { (line 158 in https://github.com/moodle/moodle/blob/MOODLE_31_STABLE/enrol/lti/classes/task/sync_grades.php). This is still the case in 3.1.3.

It could be an option to set the default for lastgrade to -1, or to add a new boolean or date field .

For Moodle 3.2 a different library is used.