We started to experience this issue after the latest ONTAP upgrade (8.1.2) to our Netapp filers. In our instance, the issue was related to de-dupe bug.
Previous version of ONTAP had an issue, where the metadata that is used to track duplicate data becomes stale. This data is supposed to be cleared out during a de-dupe run. The bug did not properly detect the stale data and left it, reporting it as "Other" on the volume and being completely invisible to all methods of perusing the datastore. After the upgrade, there was a fix installed for the bug but it did not include cleanup of the existing data. The data was then viewed as usage, after being left orphaned on the volume and messed with all sorts of things, including snapshot deltas and creating a ton of "other" space - up to 1.4TB on one volume!
A manual cleanup of the de-dupe data was required.