Have seen it twice. Once was just before realm restart (no more room in that party), the other when he was at 10% hp (same). Honestly, I'd like to see its respawn tied to Sha-kill or something like. On over-populated servers that archivement is as rare as having "Immortal" title. Most people know about tag-sharing, but not all are going to just drop the party and share it, it is a bold decition, imagine loosing your kill because of that (hotfix?).
Idk, hope blizz change their minds about that.