That’s a very good question indeed!
Normally you would need to have had some serious quality issues for a shipment to be sent all the way back like that. As the shipping costs would be quite significant, so if it was only a minor issue, the seller would almost certainly agree to steep discounts to avoid having to turn the ship around since that tanker would almost certain have had subsequent jobs lined up for after it has delivered the original shipment, which it now won’t be able to do so would need to be compensated for on top of the fees for the return trip.
It’s also very odd for the USN to show such interest in this returning tanker, as if they were trying to enforce sanctions and hurt Iran economically, it makes no sense to intercept this tanker on the return trip rather than when it was heading out to China in the first place.
Part of me wonder if the US secretly managed to sabotage the shipment and was hoping it would damage Chinese refineries if they tried to process the trap oil (remember the recent totally not suspicious refinery explosion in Russia?). With China already suffer energy shortages, a major incidence that takes one of its refineries offline for a fair amount of time would be extra damaging.
But China managed to detect the bad oil and sent it back to Iran for full investigation and the Americans were trying to seize it to prevent their plot being revealed to the world.
