Run a recover on the file and see if any problems are reported. Your file may be damaged.
Best practice is to not use recovered files, but to replace damaged files with undamaged back up copies--importing data from the recovered copy if needed to get the most up to date data.
I hear what you say, but if this is file damage, then 3 files so far are damaged in exactly the same fashion, which is mighty odd. I say so far because I haven't tested any of the others.
I think there is something more fundamental at play.
Thanks for your comments.
This kind of change in behavior will not just happen unless either a change has been made to the design or deployment of the files or they have become damaged in some way.
I'd run the recover if for no other reason than to rule out possible file corruption.