I can reproduce this problem with Firebird 1.5 and Firebird 2.0.

If I run a backup using gbak at the command line or using IBOConsole,
the OAT (of the database backed up) after the backup will be perfectly
fine. However, if I use IBOAdmin or JvUIBBackup from my application,
the OAT increases by one during the backup and the oldest transaction
will never go up afterwards without a sweep. Since the author of
IBOAdmin and IBOConsole is the same person, I suspect there must be
something I'm missing. I have tried every combination of options I
could with the components and nothing seems to help.

I have ensured all connections are closed to the database and doing a
sweep before running the backup and that did not help. I have gone
through the source code and no problems seems obvious; there are no
explicit transactions starting anywhere in either of the sources for
IBOAdmin or JvUIBBackup so it "seems" to be something that the API is
doing. Surely if this was a problem with the API, someone would have
caught it by now. Having to run a sweep after a backup all the time
does not seem like an ideal solution.

Any help would be greatly appreciated!