There are places where the lesson code incorrectly call format_text on lesson page content with urls not rewritten (urls that contain @@PLUGINFILE@@) since MDL-39490 was introduced this produce debugging developer warnings like
Before calling format_text(), the content must be processed with file_rewrite_pluginfile_urls()
We need to review all calls to format_text and were necessary call the lesson get_contents method which correctly call file_rewrite_pluginfile_urls before calling format_text