Start the registry editor on the server which has Veeam Backup & Replication 5.0.1 or later installed, then:
1. Browse to HKEY_LOCAL_MACHINE\SOFTWARE\VeeaM\Veeam Backup and Replication
2. Create DWORD = DisableVBKRename
3. Set Value = 1
4. Restart Veeam Backup Server
Perform a new full backup and then the filename of the VBK will not change anymore. This will resolve the issue.