Update 01/10/2020: After updating to Firmware 2C45, we retested the headphones and our results showed a fairly significant drop in isolation performance, primarily in the bass-range. This means that with ANC turned on, these headphones won't do nearly as good a job blocking out the low engine rumbles of planes or buses as they did before this update. This review reflects these changes.
Update 12/04/2019: After receiving multiple reports of the ANC being worse than before after the 2B588 firmware update, we retested the Apple AirPods Pro's noise isolation performance and didn't get noticeable test result differences. Apple might have tweaked the ANC, but not enough to noticeably impact the overall performance.