The docs say that the dashboard needs to be open and not be in edit mode for the auto-refresh to work (I guess the logic is that there’s no point refreshing it if nobody is looking at it!)
The autorefresh intervals begin at the time of day that you turn on this feature, and the dashboard refreshes on the interval you set as long as the dashboard is open in a browser tab and is not in edit mode. If the dashboard is closed or in edit mode during a scheduled refresh time, it won’t refresh. If the dashboard is closed or goes into edit mode, regardless of whether or not it is during a scheduled refresh time, the refresh interval restarts as soon as the dashboard is opened again or leaves edit mode.