If I sync all photos with constrain crop, I get bizarre behavior. Sometimes it refuses to display the constrain crop until I uncheck it, and then recheck it, but often, even this will fail when I export the photos (the contrain crop has no effect)
I've had this problem since lightroom 3. It seems to be better than it was but I can confirm it still happens in lightroom 4 (latest version).
What I see happen is that I'll select all photos, auto sync and turn constrain crop on and it'll occasionally leave a few photos out from being synced with constrain crop on, for no particular reason. It happens regularly enough to be a problem.
I'm also not sure if it correctly stored constrain crop in the preset too.