Markdown: Filtered content lost when editing `menu_order` without passing content.

48 out of 56 posts on my site that were previously in markdown are now only in HTML.

The `post_content_filtered` column for these posts contains HTML, not markdown as I would expect it to.

I have yet to establish a cause or pattern, or notice at what point the posts switched from markdown to HTML.

Unlike #1624 there are no shortcodes in these posts.

Please let me know if there is anything I can do to help pin down the issue.

Start by reproducing an edit that changes menu_order without passing content, then inspect how post_content_filtered changes for posts previously stored as Markdown. Compare the behavior with issue #1624 while accounting for the report that these posts contain no shortcodes. Done means the cause is identified and Markdown remains intact through the reported edit path.

Written by the indexing model from the issue text.