The arrangement of the converter has it referenced to the positive supply rail. The load as well. C28 is the typical position for a reservoir cap. However C30 also performs the same job even though it's referenced to 0v ground.
The designer could have been creating a safeguard by putting in two caps. The solenoid being an inductive load is liable to generate spikes at odd times. So depending on what polarity Is the spike, one or the other cap can absorb it, since one is referenced to supply plus, the other cap is referenced to supply minus.