On December 16th, Apple released a new firmware update for the AirPods Pro bumping the firmware from 2B588 to 2C54. After the updates, there were several anecdotal reports that the AirPods Pro noise cancelation may have been affected adversely by the update. Adding to the confusion is that Apple has since pulled the 2C54 update, so not everyone with AirPods Pro may have the latest update.
Users asked tech review site RTINGS.com to reevaluate their previous testing of the AirPods Pro with the new firmware to confirm any changes in the new firmware.
RTINGS published its findings today and does confirm that the 2C54 objectively worsens the AirPods Pro active noise canceling feature:
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.
The news is not all bad, however. In other areas of testing, the 2C54 Firmware appears to improve performance in frequency response and bass accuracy. Note: RTINGS incorrectly refers to the firmware as version 2C45, not 2C54.
Apple pulled the 2C54 firmware only days after its release, so is presumably working on the issues described. You can tell what firmware version you have by going to Settings -> General -> About -> AirPods Pro.