During the questline the caretakers says that Ysera was amongst the last souls to come to Ardenweald before the anima drought began. So she got there naturally, but shortly after that the Arbiter got shut down. Could Sylvanas' deal with Helya have something to do with it?
As for Ysera's first time in Ardenweald: well duh, she hasn't died before. She was born as a Proto-Drake in ancient Azeroth and uplifted into a Dragon Aspect by the titans several dozen millennia ago.