If your tape retention is shorter than one you have on disks, the described behaviour is expected. Backup server tries to copy all source data that is not currently present on tapes.
If you're satisfied with the described situation, you might want to switch to one of backup modes that don't require frequent full backups, such as reversed or forward forever incremental mode. In the former case, the latest point will be always a full backup that will be then copied to tapes during each tape job cycle. In the former case, this problem can be solved via virtual full backup
directly created on tapes.