Sorry, I am unable to understand the whole thing, if could attach some screenshots to illustrate, that would help.
In the meantime - As I understand, snaps were deleted? So, could you check if "Snapshot autodelete" is enabled on that volume? Might be the case where threshold has been reached and snaps deleted according to the settings. While this is investigated, you could disable "autodelete" setting to prevent snaps from being deleted.
Snapshots are deleted by SnapCenter only when the retention is hit.
If a snapshot is removed before that time, then it is either done by an admin or it is done through the snapshot autodelete option described above. If space was low then I would suspect autodelete. Read the links given above and see if that fits your environment.