It looks like it's only for old messages. My guess is that what happened is that they migrated some old messages to a new model. Maybe they had a mess of different old models, and some of them where converted to the wrong kind. I guess they were not careful enough with it, I just hope they will be able to revert it...
With any luck they'll start undeleting less-than-charitable picture comments I removed once the subjects of those comments joined Facebook. You know they still have them.
