i've made a tape backup job a while ago and it runs very stable until the U4 Update.
It's setup to never create a new media set and use the current. This is for our incremental backups.
Since the Update U4 all of the backup jobs create a new media set.
Media Pool settings:
What I tryed:
Create new Jobs
Create new Media Pool
Use other Tapes
Reboot
Job's logs may shed some light on the situation. Try to search line Need to create new media set in C:\ProgramData\Veeam\Backup\yourtapejobname\Job.yourtapejobname.TapeVmBackup.log on your VBR server. There you'll find the reason of a new media set creation.
[04.02.2019 09:27:23] <16> Info [CTapeMediaSet] Need to create new media set because media set Incremental Tape 109 has expired last medium 002010
[04.02.2019 09:27:23] <16> Info [CTapeMediaSetLockManager] Cannot use last tape media because new media set creation is required. Opening new media set
How can the media set expire ? Wich option is that exactly?
That's an unexpected behavior, job should reuse the last used tape instead of opening new media set. Confirmed that in my lab. Please, open a support ticket.