This is an unfortunate timing issue. Since the backups were at approximately the same time, the selection was done before either had completed and updated the MODBIT. The MODBIT is only reset once the data set is backed and disposed of – not at the start.
This is a general behavior for concurrently running backup steps with overlapping selection criteria.
At first CA Disk builds an internal table with the data sets by using the selection criteria, e.g.
Then the data sets are processed one after the other, that means enqueued, opened, read and copied, closed, and finally dequeued.
The MODBIT is reset after the close.
That means, a 2nd Backup that is running at the same time and checking the MODBIT, can select the data set again until the 1st job completes the Backup of that data set.