I got disconnected yesterday during withered training and when I came back I was still in the scenario with most of my withered still alive. I guess it just depends on how quickly you get back in and how many enemies were around you at the time of the d/c. I was lucky in that I had ~35 withered with me and not too many enemies around, when I came back all the enemies were gone and I still had ~30 withered.
they respawn pretty fast, took me like 10 minutes yesterday to get ~900 mana on stormrage