Attribute List (MX10)

Has anyone come across the following: (Version 9.0.1)

We have lists built in MX10 which suddenly fail to produce data. If we replicate the list and making no other changes the new list works. What I found was that the sequence number was missing on the old list (MXLISTDR - Table) So I did a change on the lines of the list, previewed (returned data again) and validated the sequence numbers now existed. We are trying to figure out why this would have happen and any insight would be appreciated.

Note: We had just copied PROD -> TRAIN and in TRAIN the sequence numbers were there so whatever happen fell within a small window.

Thanks
Robert