Sync Slicers is buggy in PowerBI Desktop (latest Oct 2018 release) - if you stop syncing, the slicer often still filters the pages it was applied to, even if you untick the boxes on the Sync Slicers properties - very frustrating. If you completely delete the synced slicer, it still applies (which is even worse)!
You can work around this (and prove its effect) by copying the visuals to another page tab where they will work (fortunately). This demonstrates the fact that there's still a hidden slicer on the page. This is a serious bug, wasting time and putting people off. Should be easy to test and fix at HQ though :-)