With ref to a mkl03z32 mcu, kds3.1 and ksdk1.3, when using the fsl_adc16 with the "Vref pair" as Voltage reference, the measure of the bandgap gives erratic measures...
It is so necessary to add the following code
PMC_REGSC |= PMC_REGSC_BGBE_MASK;
SIM_SCGC6 |= SIM_SCGC6_ADC0_MASK;
to make it operate.
Imho selecting the measure of the bandgap in the component should enable automatically the bandgap function, or at least show a warning!