Why do mass updates to post_content shows in front end, but not in edit screen?

We had someone migrate several blogs in Movable Type to WordPress recently, but we just realized they missed some of the content that was in a database field in MT labeled *entry_text_more*. For various reasons, we do not want to have the original service provider work on this; so, we are trying to fix this issue with a UPDATE… JOIN… on the old *mt_entry* table and the new *wp_posts* table in the database. (All the old and new data is in the same database, with different table prefixes.)

This command works:

UPDATE wp_posts 
  INNER JOIN mt_entry 
  ON entry_basename=post_name 
  SET post_content=CONCAT(post_content, 'n', entry_text_more) 
  WHERE entry_text_more IS NOT NULL 
    AND LENGTH(entry_text_more) > 0 
    AND entry_basename='a-sample-post-name';

(We’re just using the last AND-clause for testing. Once it’s working, we can drop that last AND-clause and do a mass-update.)

We have run this query on several individual posts, and when we view those updated posts in the front-end, or SELECT it from the *wp_posts* table in the database, the *entry_text_more* has been appended to the existing WP content.

However, when we go to Edit the post (to clean it up), only the old content appears in the Edit box, either in Visual or Text mode. And when we hit Update, the appended content disappears from the database, and of course from the front-end view as well.

We have WP-Supercache disabled, and we even tried adding this to the wp-config.php file:

define('DISABLE_CACHE', true);

but that didn’t help.

Can anyone help us figure out how to get the appended content to “stick”?