v5.05 Logical Log Backups

Anyone out here who can answer this little conundrum? One of our customers has a v5.05 database utilizing 15 10MB logical logs (I know that they need to be increased). They do not run continious log backups because they have the entire disk mirrored on a different site, so they don't care about keeping the LL's on tape. They have the tape device for the LL's set to /dev/null. My understanding of this was that as the logs become full, and once any work within them is marked as complete (ie, any BEGIN WORK's have been correctly paired with a ROLLBACK/COMMIT), the log would be cleaned and marked as free. What's actually happening is that the logs are sitting there and only a tbtape -a is actually zapping them (marking them as backed up). This works OK on other sites, so any ideas what's gone wrong here? TIA Riz