I belive this is an ancient bug from the dawn of time, or launch anyway. I had it on both my pt's lvling up way back when. I guess its a thing when he is avilable for a while and then not and then again not being made avilable after the q series on Taris is done.
If i remember correctly; after i started next planet i could press 'n' and summon him, after that it worked from the ui as well.