I am experiencing this as well. On one specific high traffic course at least. Suddenly Moodle has started to skip over some completions for quizzes that require a grade to pass. In some cases Moodle doesn't even seem to attempt awarding the completion. Here are some log snippets from two different users. *This is in a 3.9.3 site*
This one shows the 'completion updated' step followed by submission. Which is all proper.
Here is another user in the same quiz getting the same exact grade; The completion step doesn't occur.
If I unlock completions for the quiz Moodle will successfully go back and award all completions even for users affected by this issue. What could be causing Moodle to skip over completions for some users?. Could it have something to do with the completion state in the database?.