Ok, so assuming that they know the job well. From my knowledge, flushing is done, by adding the flush fluid, then draining out the same fluid out. Again adding the flush fluid, then idling the engine a bit and then flushing it out, then topping up with oil. (I am not sure, but I am confident that, there will be a slight idling, so as to circulate the fluid along the components). Was this done?
Another thing is when I gave my car for servicing my car already had Liqui Moly super diesel additive in my fuel tank.