I had this problem as well - so my solution ( so I was told by others ) was :-
When the Harvesters are at the Gas Giant and mining Sorium , set the destination as your planet and the Primary Condition / Conditional Order to Fuel Tanks Full and Harvester Transfer and Return.
What should then happen is the Fleet should return to base , unload , return to the Gas Giant then commence the cycle again when full ( and so on ).
DavidR