I've had it work fine for me but I always researched the tech first before making the refueling ship. Maybe that matters?
Do you click the the drop down box in the fleet management screen and set the tanker to refuel same fleet?
I do not remember for my first generation of tankers, but I definitely had the 30% underway tech when I designed and built my second generation of tankers.
Yes I did set my tankers to 'refuel same fleet'. They refuel the fleet just fine but only if the fleet is idle.
The fleets I have tried to refuel have all been geosurvey ships that ran out of fuel. They are 'underway' only in the sense that they have an active move order. I have never tried to top up a moving ship with a partially full tank. Maybe that distinction is the problem.