Capacitor loading lets Atmega4809 die
A diode from GND pointing to 5V is a good idea.  Also, resistors in series with the RX and TC (maybe 270 ohms).  When you connect the BLDC, perhaps it is actually 'bouncing connection' with many small spikes over a few milliseconds.  With these high currents you will get spikes of a few hundred volts lasting only micro-seconds.  These will be discharging wherever they can.  The diode from ground to 5V should help, the resistors are on the TX and RX are an extra precaution.