I found 2 lab cases which reported this issue, and both are saying about the same thing
From the 'DB2" integration Guide
Archived logs are automatically backed up whenever a new offline redo log appears, for example, after the online backup of DB2 objects completes.
Therefore, do not start an online backup of DB2 objects before creating an archived logs backup specification.
Delete any old archived logs backup specification before creating a new one.
and, from the other lab case:
To run the backup of archive logs, the user has to create a seperate barlist by selecting the archive during creation of barlist and the entry for this will be upadted automatically in the db2 config file for that database respectively.