Buck Converter Bad Regulation
As already mentioned in the comments and stated in the datasheet of this converter, a possible problem would be the feedback network. You should reduce the feedback resistance, or make sure that the layout accounts for undesirable effects.
According to the datasheet:

The choice of the RFBT depends on the application. RFBT in the range from 10kΩ to 100 kΩ is recommended for most applications. A lower RFBT value can be used if static loading is desired to reduce VOUT offset in PFM operation. Lower RFBT will reduce efficiency at very light load. Less static current goes through a  larger RFBT and might be more desirable when light load efficiency is critical. But RFBT larger than 1MΩ is not recommended because it makes the feedback path more susceptible to noise. Larger RFBT value requires more carefully designed feedback path on the PCB....