I heard from another customer that upgrading to ONTAP 9.1 solved the issue. If you can't upgrade, the only other workaround I can suggest is to update any graph panels that show the volume 'write_data' metric and instead make it the 'qos_write_data' metric which is functional.
Hope this helps!
Cheers, Chris Madden
Solution Architect - 3rd Platform - Systems Engineering NetApp EMEA (and author of Harvest)