I have not read of anyone reporting this issue before. And you did not mention any issues like an uneven idle.

Are any of the hoses in the return kinked even partially?

I have read of the interior of brake lines deteriorating and blocking off flow but never the return lines. As mentioned with the little nipples on the Injectors maybe there is one of the nipples the return line goes on that is nicked/scratched or even partially blocked off

You could get a longer hose to test (connect it to the fuel return) and fill a jar 1/2 with diesel fuel and submerge and see if there is bubbles coming out of the return.

This sounds bizarre but if 2 parts of your Injector Nozzles are not seating well it allows combustion gasses to pass into the Injector body when the fuel is not being injected (and to a lesser extent air from compression). Somewhat like having a bad valve in your cylinder head. Combustion gasses can get by a bad valve seating area.

This does not fit your particular issue but other people read these threads. Swapping the position of the Injector to see if the leak is still on the same injector could be revealing.While I was writing the above I forgot about this. The little return nipples are pressed onto the Injector Body and swedged/crimped over to seal them tight. However, once in a while those nipples become loose and it is not the hose that is leaking but around the nipple. Putting a scratch near that possible leaking nipple so you can identify it and swapping the position of the injectors to see of the leak follows the same injector would be an indication that the leaking nipple is either loose or has some other issue.

The normal fix for the nipple issue is to replace the upper Injector Body but be aware that doing that changes the pop/opening pressure.

One of our members solved the issue of a loose return nipple buy cutting it off and welding over the hole and he always kept that Injector at the rear.

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84 300D, 82 Volvo 244Gl Diesel