From Janauary 28 update it is now possible to return to Mistlock Observatory in the middle of a Fractal Dungeon run, as you can check in this screenshot:
What I would like to make sure is: when you travel back to the observatory, does it reset all fractals did until the moment or the current one? Or does it gets you back to the last checkpoint?
This is not new to the update, and has been an option since Fractals of the Mist started.
Think of "Return to the Observatory" as a "Quit" button - it kicks you out of the current Fractal, and, if you start up again, you will start with the first fractal in the cycle. You will not gain a difficulty level, though you'll keep anything you've already looted.
If you want to return to the last checkpoint, you can do so when all of your party members have been defeated.
