I have a similar situation. Backup exec, by default, grabs any media that is allowable for overwrite, even if it takes it from one mediaset and changes it into another.
I fixed this by changing the setting located at:
Find the setting under the menu "Tools->Options->Settings/Media Management"
click "Overwrite recyclable media contained in the target media set before overwriting scratch media"
click "OK"
Since a tape that is allowed to be overwritable is considered "scratch media", this will cause Backup Exec to use recyclable media instead, thus keeping tapes from hopping mediasets.
Also, if you have a carousel, you can partition it and set Backup Exec to backup to the specific partitioned slots, which also helps to make sure media sets never hop between each other.