Say you have a quiz using question types that don't support the Moodle app (e.g. qtype_multichoiceset). A student attempts the quiz through the browser and fills out all questions, but doesn't submit the attempt yet. Later that person opens the quiz again in the app.

At this point Moodle creates a new attempt, but copies the values of the last attempt only for those question types that support the Moodle app. The student doesn't touch anything so when the deadline passes, only some results are submitted. The questions not supported by the app are treated as unanswered, even though the student may have answered them correctly before.

What's even more confusing: when you look at the results (at least in the case of qtype_multichoiceset) it shows that you entered the correct solution but tells you it's wrong. See attached Screenshot.