This is similar to issue: MDL-26442

A client of ours reported a bug when attempting to either:

• Duplicate a particular existing quiz
• Backup then restore a specific quiz into new course (error only occurs on the second restore of the same backup file)

Error shown to the browser is: error_qtype_match_subquestion_missing_in_db.
This code relates to the process_match method of restore_qtype_match_plugin.class.php

The problem in this instance is that the $this->questionsubcache array does not contain the question text. The value of $data->questiontext is  "<p>Self- rating of how important <a title="Physical attractiveness" href="$@QUIZVIEWBYID*419@$">physical attractiveness</a> of a romantic partner is to them<br></p>"

While the relevant array item is the same except for the href="" url which contains a proper url, ending in /mod/quiz/view.php?id=419

Because of this, the quiz question cannot be matched.

I see the backup_quiz_activity_task::encode_content_links method to replace /mod/quiz/view.php?id=[0-9]+ with string $@QUIZVIEWBYID*$2@$ - but it appears the inverse of this should occur before the array is searched in process_match().

I'm not sure how the client has got the data into this situation. However, if I create a backup of the activity then restore it into a local vanilla Moodle twice, I get the issue. If I then delete every question in my local Moodle quiz activity then restore the file, the issue persists.

Here is the backup of the activity:
https://drive.google.com/file/d/1cGxogd2U2kCpQJm1pYD-KwhuHLlrAezn/view?usp=sharing

I'm not sure why this is happening, but this page advised me to report this here:
https://docs.moodle.org/35/en/error/moodle/error_question_answers_missing_in_db