Error Message

SGMON Debug logs:

SQLSERVR: [0000] BECAT : [:707] : SPID(67) : [CATDLM_GetExpirableImage_BE] [Stage 3].[Proc] : Unexpired or retained dependents @count = 1

This will happen if not all of the backup sets (Full, Incremental etc) on disk storage device(s) were duplicated to tape storage. 

If all the sets from the Full and Incrementals etc have been duplicated the disk sets will not show a dependency on the tape sets and DLM will work as expected

This behavior is by design. 

Backup Exec keeps expired Full and incremental Disk backup sets for restore purpose until each set in the recovery chain is expired. Since Tape backup sets are managed by Advance Device and Media Management (ADAMM), expired sets on Disk will still be available on media server. DLM needs to keep expired Full and Incremental sets until the tape sets are overwritten and hence may show as dependent set for expired disk backup set. 

As an example lets say that a job has weekly full and weekday incremental jobs which are each duplicated to tape. If the full backup set and incremental sets all have been duplicated to tape, this problem will not happen. If one of the incremental sets did not get duplicated to tape, the disk sets needs to be retained. Otherwise not all the sets needed for a restore are available. The missing set for the complete recovery chain are on disk and is retained to ensure that all the backup sets can be restored from.

Customers using Backup to Disk storage and Duplicate to Tape storage strategy should ensure each disk backup set (Full and Incremental) are duplicated to Tape storage to avoid this situation so expired disk backup sets will not show any tape sets as dependent. 
If only full backups are required to be duplicated, it is recommended to instead create a separate Full backup job writing to tape, so the problem described here is not encountered.