Unreal Engine does not use fixed time steps, and cannot get deterministic behavior using the default physics integration. The best you can do is enable sub-stepping, setting some small sub-step size, and hope for the best. I have a simulation that definitely “hitches” when something takes away the machine for a few milliseconds 
To provide consistent (but still not 100% deterministic) behavior with variable time step, you’d have to use a Runge-Kutta-4 integrator, which is way too expensive (calculating all those derivatives) and also use swept collision detection across the board.