I never tried using Fuel Harvesters as tankers, so I'm not sure. To be clear, I mean my fuel harvesters park and harvest fuel. My tankers (which do not have harvesting modules) load from the harvesters and unload at a colony.
I'm not sure anybody else has used ships equipped with harvesting modules to ferry fuel back and forth.
It is also unclear to me if your problem is that the 'tankers' are trying to give fuel to the 'harvesters' (and failing because they are full) or they are failing to take fuel from them (and failing for some other reason).
As a workaround, spead out each ship in its own subfleet, and experiment with conditional orders to "unload at nearest colony when full" and direct orders to do the same and various combinations of 'refuel A from B' and 'join B and refuel fleet' etc.