Any fixes to this? Yes, I am on Xbox one.
The lancer initiates are vertibird pilots, and glitches sometimes cause them to go walkabout, but because the only way to kill them is destroy their vertibird, they are totally invincible.
Try reloading a previous save before you had this problem, or failing that, fast travel somewhere far away, save, quit, then restart the game. Fast travel back to the settlement and he might be gone.