LT8641 FB voltage higher than 0.81V

From the datasheet, the FB voltage is a function of the VOUT and the voltage divider. The regulation can be affected by noises getting into the FB and GND pin, noise coupled through the capacitors and ripple voltages. Good layout and component selection can minimize these. You can check for layout considerations and component selection on the Applications Information section of the datasheet.

Regards,

Nino

Thank you.

Another question, for the BIAS pin, if I tie this to a 3.3V power supply without the recommended bypass 1uF capacitor, would it have any effect? 

Bypass caps are used to minimize the loop to reduce the noise. If the noise is severe enough, it might damage the part or make it not work properly or both. I highly suggest following the datasheet recommendations and guidelines.

Regards,

Jhun

Bypass caps are used to minimize the loop to reduce the noise. If the noise is severe enough, it might damage the part or make it not work properly or both. I highly suggest following the datasheet recommendations and guidelines.

Regards,

Jhun