In feedback systems, the gain is a function of frequency, and typically decreases when going from low frequency to high frequency. This is often accompanied by a phase delay.
So if the overall gain of the system is high enough, there will be some high frequency where the gain is 1, and the phase is 180 degrees. This would result in positive feedback, amplifying noise at that frequency.
Maybe that’s what’s happening in the latest AirPods? If Apple is aggressive cranking up the gain of the noise cancellation system, there’s some high frequency where the noise gets amplified rather than suppressed.
The solution would be to either reduce the gain (which reduces the noise cancellation), or to add some differential gain in the system which pushes out the unity gain frequency to higher frequencies.