One of my colleagues is working on it, so I don't have all the details.
However, it seems to be that students are adding images in quiz question answers and then they cease to be visible. When we investigated we found this Base64 data.
* purge caches
* Go to Site admin > Server > Tasks. Check the tasks page and the logs page, make sure the completion stuff is actually running
* Failing that, click the 'Unlock completion settings' button. Sometimes completion gets "stuck". Consider that this will re-complete everyone in the course. This may or may not matter to you.