Sometimes the "The Festering Core" Strike does not load the last checkpoint and therefore does not spawn in the final boss. Going back in the strike is not helpful either, because nothing happens to the checkpoint. If a new player then joins per strike playlist it spawns the player at the start. Running throug the strike changes the objective from "Investigate the Vex Activity" to "Descend into the Pyramidion" but thats it. It would be nice if this bug would be fixed. It exists for a long time.