This problem began since the January 19? update to the ProofreadPage extension. Detailed discussions of this topic can be found here:
https://en.wikisource.org/wiki/User_talk:George_Orwell_III#Adobe_Flash_and_Wikisource.3F_The_end_of_wit and here:
https://en.wikisource.org/wiki/User_talk:George_Orwell_III#It_seems_the_PR_image_bug_is_resolved_.28nope.29.
Please see these screenshots on en. Wikisource:
https://en.wikisource.org/wiki/File:Side_by_side_editing_not_functoning_properly_either.jpg
https://en.wikisource.org/wiki/File:Lost_scan.jpg
This may relate to:
https://phabricator.wikimedia.org/T59948