Execute grant for MySQL stored procedure revoked when stored procedure is altered
This happens because the stored procedure is actually dropped and recreated.  When the procedure is dropped it loses all its permissions.  Therefore, when the 'altered' version is created there are no permissions attached.

Your process to update the procedure needs to include a re-grant of the needed permissions.

I don't know your full environment, but here was a 2013 forum discussion for one MySQL tool set:
http://code.google.com/p/heidisql/issues/detail?id=3300