i had this bug too. i think it’s due to the mission objectives.
there’s one objective named “Escape with the scabbard” and another one named “Pick up the scabbard”. If one gets the former before the later one, it bugs out(for me at least).
The way it worked for me was: pick up relics, even if you already know where the scabbard is, wait for the “Pick scabbard up” objective to pop up, pick it up, escape and done.
oh, and i’m also an engineer ^^