Hmm I have not specifically checked what happens but as I believe the Overwrite protection start is based on last write to a tape and not specifically the end of the job, then for spanned sets there will be a difference between when first tape becomes overwritable and when the second tape does i.e. I believe OPP works like this with spanned tapes:
Scenario:
Media Set OPP is set to 7 days
Day 1: Backup Job starts and overwrites first tape, which it fills after 8 hours and then uses a second tape for 2 Hours (which it does not fill) Overwritable status of tapes at this point is:
Tape1 - Overwrite protected until Day1 job start time + 8 Hours + 7 days
Tape2 - Overwrite protected until Day1 job start time + 8 Hours + 2 Hours + 7 days
Day 2: Another Backup job starts against same media set as Day1, at same start time as Day1, is configured to append and uses the remaining space on the second tape (taking 6 hours), before going to a 3rd tape which it also fills (taking 8 hours), before writing to 4th tape for 1 hour. Overwritable status of tapes at this point is:
Tape1 - same as at end of Day1 job processing - so in effect 1 day closer to being overwritable against the current time
Tape2 - Overwrite protected until Day2 job Start time + 6 hours + 7 days
- in effect this is now close to a day further away when measured against Tape1
Tape3 - Overwrite protected until Day2 Job Start time + 6 hours + 8 hours + 7 days
Tape4 - Overwrite protected until Day2 Job Start time + 6 hours + 8 hours + 1 hour + 7 days
As you can see there are always differences between when tapes can be overwritten in a spanned set - and these differences can be added to by using append jobs which can move the overwrite protection of the last tape that is not completely full further away from the earlier tapes.
There is however no way to configure it such that the overwrite protection for Tape1 also moves backwards against Tape2, if its overwrite period changes. Your overwrite protection should be based on how long you must keep that backup set for and as soon as the earlier tapes in a backup set have been overwritten, you need to be aware that any remains of the backup set on a tape that has not been overwritten is a partial set with limited restore capabilitities (for file system backups) or NO restore capabilities (for GRT backups)
Usually this is not an issue for tape based backups - as backup sets are typically kept both offline (so no overwrite possible) and for long enough on tape so more than one newer backup set of the same data already exists.
We did realize that this concept is a huge problem for disk based backup sets which is one of the reasons why we separated disk based sets from Media Set handling and introduced DLM (in Backup Exec 2012 and later)