Using a DC / DC Converter?
I suspect that the DC/DC converter module requires a minimum load.  Notice that the regulation is specified for 20% to 100% load.  But the datasheet doesn't say what happens when load is less than 20%.

You can check this hypothesis, connect a 100Ω dummy load resistor (it should be rated for 0.25W or more), and measure the output voltage.

 (source: p.2 in the datasheet)