I found another post which might be correlated (I have had anti spark plugs too, I did remove them now). Maybe that is an important finding that anti spark plugs are dangerous.
Another question would be if a temperature change inside the case with the flight controller could cause such behavior. I have a Herelink Airunit which heats up the copter quite significantly. Maybe a fast start could mean that there is a huge temperature difference between initializing and flight.