I just had the same issue. The problem was that I had a zoom action set up in the list, so adding the URL parameter to zoom also meant that the dashboard had two map zoom actions. Removing it from the list made the zoom work. The bug is here: BUG-000148627 for ArcGIS Dashboards (esri.com)
In my case, I made a copy of the dashboard to use just for the URL parameters.
I hope this helps!
-Kevin