As a feedback, I checked if the same app can run S.O.S mode in background, The result was No it can't just the steady flash mode only.
I'd wager that what you're observing is nothing more than W10M no longer automatically turning the light back off, when the app that turned it on is suspended (no longer in the foreground). That means the flashlight app isn't running in the background on W10M any more than it was on WP8.1. That would also explain why the flashlight app can't blink S.O.S. when not in the foreground.
That's just a guess, but I'd bet on it...
Anyway, I just need to add that If you tried W10M with all your knowledge, I think you will notice that there is a change on the way it handle background tasks. Even I can download files in background with IE or Spartan, In addition when you playing youtube video and press the Start button you will notice that it still running.
I'm looking forward to doing so... maybe the next preview of W10M.
Here again though, I'd like to stress that what you're seeing is
NOT a change in how W10M handles background tasks or multitasking. I think we'll see very few changes in that area... if any! What you are seeing,
which has been my main point all along in this thread, are improvements to apps!
UC Browser has long been able to download files in the background on WP8.x, and as you say, Spartan on W10M can now do so too (finally!). That's not the OS that is changing, but the browser!
Metrotube has long been able to continue playing YouTube videos in the background on WP8.x, but IE couldn't. I suspect you're saying Spartan can now do that too. Again... it's the browser that is changing, not the OS' multitasking capabilities.
Overall, I don't care with "Multitasking" or "Background Tasks" Technical Definition, As a customer the thing I need is to operate all the apps I need at the same time and go from that app to another with the multitask screen and this is what in W10M Already. All the apps you opened will be on this screen. I go from Word App to Spartan To Outlook To Facebook To whatsapp as all are opened.
Nor should you have to care. On the other hand, this is an enthusiast site where many of its members are at least somewhat interested in how these things actually work, so I don't think my explanations are completely in vain.
I'm not trying to turn everyone here into engineers. However, I do want to help those who are interested in gaining a better understanding of what is going on and help prevent FUD from spreading in the WCentral community.