I think the only certainty is the injection pump internally produces very strong pulses in operation. The cigar hose obviously moderates them when the relief valve is open. With the fuel supply at proper pressure the relief valve would always be open. That's if the lift pump is strong and the fuel filters not too obstructed.

Also when replacing the cigar hose with normal hose try to get a piece of clear hose in there as it can aid troubleshooting down the road. Remember as well that information from Mercedes may not be all that good. They neither designed or produced that fuel system.

Many European versions where produced without a cigar hose. The injection pump may be different on them but fundamentally operates the same. As a really wild guess I wonder if those pulses might transit the return system and somehow have a small effect on the injectors. I guess that would depend on their peak pressure. The pulses are of extremely short duration and cannot accumulate base pressure it seems. More like a very high frequency hammering as revolutions build up.