Just to add some Blue to this, we have seen an issue that causes Outlaw Rogues to be teleported to another part of the Firelands after using Killing Spree during the Ragnaros fight of the Anniversary Event.
It’s something the WoW Team are looking into and working on a fix for, in the meantime another spec or simply not using Killing Spree in that encounter should let you finish it off.
Sorry for any hassle there, will try update again with any news but keep an eye on the Hotfix posts too incase it’s posted there before I get a chance to share it: