DC/DC Converter strange output
It is an unregulated isolated supply.

If you have a look at the datasheet they show load and line regulation only starting at 10% load.

If you have no load present (a multimeter poses no load), unregulated isolated supplies tend to increase the voltage. Depending on the design the increase can be quite substantial.

So to measure the voltage, you should present a minimal load. How little is enough depends on the supply. As the datasheet starts at 10%, that might be a good point, but maybe 5% would be enough already.

Note though, that even regulated supplies might need a minimal load for the regulation to function, so it's not necessarily different with no load present.