I thought I had this bug too, I used a blood stone in Homestead, picked up the inert stone and went looking for Zixzax. He was standing in front of an inactive portal spouting his lines to enter it. I then ported out and back to Homestead and this time on arrival got the conversation with Zixzax wherein he reveals a bit more of your former past as a Guardian. Then he ran off to the previously inactive portal which was now active. Come to think of it, Zixzax always talks to you first about your past before activating a portal, for some reason in this particular instance he didn't. But it's all okay now.