When we had the Service Station, it was made quite clear to us by the Gas supplier contract that any incorrect supply of Autogas to household cylinders would lead to instant termination of contract.
At the time, Autogas was the only product with a decent margin, it wasnt worth the risk.
Filling partially full cylinders was both a pita and time consuming. Not worth the hastles. Thats why swap and go is popular, no staff training and no risk of injury (gas burns arnt nice).