Flatcram Mites ( that generally travel with Flatcram Eels) have dropped usable leather since launch of the expansion, as confirmed with WoWHead article https://www.wowhead.com/npc=223491/flatcram-mite#skinning
As of about 2-3 days ago they have only been dropping the ‘junk’ leather [Unstable Fragmentes]. This change happened while I was in the process of skinning them. Several previous spawns all dropped [Stormcharged Leather], with a 100% drop rate as far as I could determine, then after one particular spawn, it changed to a 100% drop rate for [Unstable Fragments] and no [Flatcram Mite] has dropped usable leather when skinning them since.