Users asked the RTINGS.com technical review site to reassess their previous AirPods Pro tests with the new firmware to confirm any changes to the new firmware.
RTINGS published its findings today and confirms that the 2C54 firmware objectively worsens the AirPods Pro active noise cancellation function:
Update 10/01/2020: After updating to the 2C45 firmware, we re-tested the headphones and our results showed a rather significant drop in insulation performance, mainly in the bass range. This means that with ANC activated, these headphones will not do such a good job blocking the low noises of engines or airplanes as they did before this update. This review reflects these changes.
However, the news is not all bad. In other test areas, the 2C54 firmware seems to improve performance in frequency response and bass accuracy. Note: RTINGS incorrectly refers to firmware as version 2C45, not 2C54.
Apple removed the 2C54 firmware only a few days after its release, so it is presumably working on the problems described. You can know what firmware version you have by accessing Settings -> General -> About -> AirPods Pro.