I've been playing through all the past seasons on my Hunter. I had already done the Season of the Wish storyline on my Hunter, and part of the story is that Crow goes into the portal in the traveler and disappears from being an NPC in the H.E.L.M. Once you complete the 'We Stand Unbroken' Storyline for Season of Defiance, it gives you a milestone (Legacy: Retribution) that requires you to talk to Crow in the H.E.L.M., but he is not there and there is no way to complete the milestone. Something I have seen other people suggest was to join a friend who hasn't completed Season of the Wish, as social spaces are instanced, and talk to Crow there. That does not work, Crow will be there for my friend but not for me.