How to prevent Octave Jumping in ACF of Speech Signal?
Octave jumps down can occur if there is more noise (or other modulation) around a sub-harmonic of the pitch frequency, than at pitch.

Octave jumps upwards can occur if the timbre contains strong overtone or harmonic sequences and weak or nearly missing spectrum at or around the fundamental pitch frequency.  Noise, or a non-flat frequency response in the acoustic environment or channel, can also add power to an overtone peak.

Windowing effects in the ACF can add noise which can increase the probability of the above estimation errors.

Humans might be infering octave by using using preceding or following pitch, timbres, and transients to error correct spurious jumps in the time-local spectrum.