After doing a data update, the 'Last Full Fiscal Year' gets reset to 0.
From our experience this happens when trying to update data for a company that is not in our database.
If this has already happened, unfortunately the fastest way we have found to correct effected stocks is to start a new stock study for the stock(s) in question.
In order to avoid this in the future, the best thing we have found is that when you perform a data update, and receive a warning about the company not being found, or about a data discrepancy, choose the 'Do not update this company' option.
Further, if you have multiple companies that are not in the database, you can click the 'Use same choice for all' check-box before clicking OK.
This will ensure that the date for the company does not get reset to 0 on the Annual Data tab.