That’s normal. ext4, the filesystem used inside of VM volumes, does not keep track of already trimmed space across mounts. I think it wouldn’t even make sense to do that, because between mounts you might have imaged the filesystem and transferred it to different storage hardware. So it redoes the trimming, just in case.
If you’re not seeing an immediate increase in free disk space in the storage pool, it’s usually due to old revisions of the volume still being present on disk. You can check how many revisions are kept using qvm-volume info VMNAME:private. Shutting down the VM will delete the oldest revision and create a new one from the now trimmed data.