If you're using PVE and you are backing up your VMs with Proxmox, either "manually" or with Proxmox Backup Server (PBS), there are some catches - For example: When Proxmox uses the QEMU guest agents to run fsfreeze, it requests a full backup from the SQL VSS writer. That breaks the backup set for all databases because it consolidates / deletes the logs that are needed to restore.
So here's what you need to do to create consistent Proxmox backups that don't break your regular MSSQL backups.