This looks to be a bug with Facebook itself that is causing issues on only a few sites. We're not sure what the cause is, but it doesn't look like there's anything we can do to fix it. For more details, see:
developers.facebook.com/bugs/1436819156545934
I would recommend using the "Does this repro describe your issue?" button and adding your app to the list of those affected. Using that link will increase the number of affected sites, which is how Facebook prioritizes fixes.
I hope that helps,
Alex
Posting in Facebook with empty message
