From what I have read it has been kind of hard to pick out what people have done wrong.
I suspect 2 possible issues.
One is the actual Delivery Valve; 2 pieces the Valve in the Center and the outer Body. The Valve can only go on one side of the Body and there is a band machined into the Body so you can tell which way it was supposed to go.
But, if you remove it you have to watch which way it came out and Write it down or take a picture.

The other issues seems to be that somehow people are managing to pull up the Barrel of the Element up and out of the alighment Slot/Pin.
That can happen if you pull up on the outer barrel of the Delivery Valve, use a Magnet on the Delivery Valve or rotate the Fuel Injection Pump with the Delivery Valve Holder removed.
Depending on the position of the Fuel Injection Pump it might also happen if you jarred the Pump hard on the work Bench.

If you are working with the IP still on the Engine I would resist the temptation to remove all of the Delivery Valve Holder at one time and try to replace only one Crush Washer and O-ring at a time.

Screw the Delivery Valve Holder down by hand (if you lube the O-ring you should have no issues screwing it in) and before yout Torque it lay a staright edge acrossed the tops of the Delivery Valve to see if the one you worked on is sticking up higher than the others.

If it is sticking up a lot higher then you know you have a problem inside.

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84 300D, 82 Volvo 244Gl Diesel