Yes, you've discovered a new feature. If you are using PRESCHEDULECMD,
then presumably you have some operation that you wish to complete prior to
the scheduled TSM operation (i.e. shut down a database before incremental
backup runs). In 5.1, we changed PRESCHEDULECMD so that if the command
does end with return code 0, then the scheduled operation will not run.
This change was based on user requirements.