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)!
Further details at https://community.powerbi.com/t5/Desktop/Problems-with-Slicers-after-using-option-quot-Sync-Slicers-... and https://community.powerbi.com/t5/Desktop/Filter-remains-after-removing-Synced-Slicer-from-a-page-wit...
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 🙂