These multirotor autopilots sacrifice altitude control for the inner control loops which provide aircraft attitude stability through differential thrust. A loss or partial loss of power to one or more motors in an autopilot mode will cause the remaining motors to reduce power and the aircraft will descend. Of course the most likely scenario is the machine immediately loses stability and crashes.
The wreckage will be found very close to where the original issue occurred in either case.
The likelihood of a "fly away" event due to getting hit by fireworks is so remote it does not need additional control measures to reduce that risk further.