After loading up the Orbital Cargo module, discharging the payload & returning home, duplicants will not reload the module with the specified resource.
Duplicants will only fill the module again if either 1) The specified resource is changed, or 2) the quantity of resource is changed. Even just clicking the slider and leaving the amount the same is sufficient.
This has to be a bug. It makes it so you can't automate deliveries with it, so you either have to wait till late game interplanetary launchers, or land on the planet & have the dupes move it all off the rocket by hand.
1) build Orbital Cargo Module
2) Specify item & mass you'd like to fill the module with
3) Launch rocket to orbit, deliver payload
4) Land anywhere
duplicants will now not begin to reload the module.