The following developer debug can be found when previewing a restored quiz.
Michael de Raadt first picked this up while testing MDL-39505 and we've related it to the just integrated MDL-39490.

Before calling format_text(), the content must be processed with file_rewrite_pluginfile_urls()

line 1201 of \lib\weblib.php: call to debugging()
line 320 of \question\type\questionbase.php: call to format_text()
line 70 of \question\type\multichoice\question.php: call to question_definition->html_to_text()
line 267 of \question\behaviour\behaviourbase.php: call to qtype_multichoice_base->get_question_summary()
line 898 of \question\engine\questionattempt.php: call to question_behaviour->get_question_summary()
line 481 of \question\engine\questionusage.php: call to question_attempt->start()
line 212 of \mod\quiz\startattempt.php: call to question_usage_by_activity->start_all_questions()

Before calling format_text(), the content must be processed with file_rewrite_pluginfile_urls()

line 1201 of \lib\weblib.php: call to debugging()
line 320 of \question\type\questionbase.php: call to format_text()
line 210 of \question\type\questionbase.php: call to question_definition->html_to_text()
line 267 of \question\behaviour\behaviourbase.php: call to question_definition->get_question_summary()
line 898 of \question\engine\questionattempt.php: call to question_behaviour->get_question_summary()
line 481 of \question\engine\questionusage.php: call to question_attempt->start()
line 212 of \mod\quiz\startattempt.php: call to question_usage_by_activity->start_all_questions()