Many Safari users have reported that YouTube playlists don't autoplay when the tab is in the background and AdBlock is running. Autoplay works fine with AdBlock paused.
We have had limited success in reproducing this issue. However, there is a workaround. Just drag your YouTube tab out of Safari so that it's running in a separate window, with no other tabs open in the same window. Then YouTube autoplay should work for you with that window in the background.