Formatting Changed on Existing previous posts

Formatting issues became apparent when 3.9.1 was installed. Paragraphs disappeared, formatting of lines disappeared, all jammed together as in one big paragraph.
Some of the posts were repairable just by opening the editor to edit, and “updating” the post. Most did not improve.
NOW, with Version 3.9.2 the formatting can be improved by double-spacing and placing H2 lines between paragraphs, but otherwise it’s the same problem.

What’s the fix? The submit box (editor) is clearly not working properly.