Information regarding BOD reset and other reset sources can be found in Reset Register Status.
On FSFEB, the BOD threshold is set to 2.87V. This means that when the voltage goes below 2.87V, the BOD will hold the device in reset until the voltage is above 2.87V again.
The BOD LED is connected to the board controller. Since ATtiny3217 is in BOD reset when this LED should be ON, it cannot be controlled by ATtiny3217 itself.