I just spotted this comment on WoWhead:
"There are at least two confirmed spots according to Thottbot and a few comments here. One on the cliff southwest of Grizzlemaw at 47.47 and the other at Amberpine by the River at 30.56, which seems to be the popular sighting spot." While the original Thottbot comments/locations are missing (it's since been merged with WoWhead), I've just gone out there and had a look, and it's definitely not a place players would often visit--it's a small cliff nestled against the giant log, overlooking the furbolg battle, just south of the double "z"s in Grizzlemaw.
So, if anyone is out and about looking for Arcturis, can't hurt to check this second spot; maybe there really is another spawn.