The really odd thing is that I can restore the post like this:
Click on Quick Quote. Copy the post. Click on Edit. Paste in the post and remove the quotes. Change from using UBBCode to using HTML and UBBCode, and the post is restored.
And sure enough, when I select POST_BODY there is content there as well as in POST_DEFAULT_BODY.
Well, it's not really odd at all, quick quote pulls the unparsed content from the "POST_DEFAULT_BODY" field, whereas everything else pulls the parsed content of the "POST_BODY" field; by re-filling the data out you're simply re-populating the field with new data.
Is this behavior being seen everywhere when a new topic is created, or topics by a specific user, or are they spurratic (IE only showing up whenever it seemingly wants to show up)?