UCG28826: Why brownout protection senses the Vbulk, not Vin?

Part Number: UCG28826

Tool/software:

Hi TI,

As title question, it cause output voltage discharge very slow at light load and some timing issue on system. 

See below waveform, after AC OFF, the PWM is still switching until Vbulk drops to low. 

As my known, brownout should sense the input Vac.   

CH1:Vbulk CH2: input VAC  CH3: SW pin