The presence or absence of a feeder/branch grounded conductor does not change the system. The creation of an ungrounded or grounded system can only take place at the source of the system which is either a transformer secondary or a generator output.
What are your measured line-line and line-ground voltages at the "power" service and the "lighting" service. Ideally you would take these mesurements as close to the same time as possible.
Can you put a recording volt meter at the load to see how the voltage changes over the course of the day?
