I mean Allow, Deny or Ask? Or you are using that option without any exception?
My default option is "Click to play" (which corresponds to "Ask" in Exceptions). My exception for youtube is "Detect content" (which corresponds to "Detect and run important plug-in content (recommended)" in default settings).
So, I'm using "Click to play" as default, and "Detect content" as youtube exception.
(Btw, different terminoligies for default vs exception settings, for same functionality, is counterintuitive, and I feel that noobs can get pretty confused)
Well, i installed an extension to force Flash player on Youtube, ran a playlist and the videos played automatically even without the tab being the focused one.
I tried yet gain and same outcome. refused to start playing until I focused the tab.
Additionally, like yesterday, in order to rule out extension/userscript conflicts, I also allowed DomainUserAgent to run in private mode and then opened a new private window with 2+ tabs. In one tab I went to a random yt playlist, and immediately switched tabs before it had loaded. The outcome was the same: demanded me to focus the yt tab in order to start playing. Also moved the video position to relatively near the end of the video in the playlist, and switched tab and waited. When the video was finished I could see the tab reload, and then I waited and waited and waited... and nothing happened. I again had to switch to / focus the yt tab for the video to start.
May I ask what extension you used to force flash ?