Via MacRumors, RTINGS.com published updates to their AirPods Pro analysis, in response to firmware update 2C54.

Follow the headline link and search for the word Update. There are a number of them, all dated 1/10/2020.

The most significant negative:

After updating to Firmware 2C54, 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.

From the MacRumors post:

Apple pulled the 2C54 firmware only days after its release, so it is presumably working on the issues described. You can tell what firmware version you have by going to Settings -> General -> About -> AirPods Pro.

UPDATE: A Loop reader offered this comment:

In my opinion, the difference is huge! I initially thought my AirPods were not working well. Many times a day I would me moving them in my ears to see if I could get a better fit. Grabbed my dad’s AirPod Pros, and the noise cancellation was amazing.

Today with the news, I checked my AirPods, and saw I had the bad update. Checked my dad’s AirPods, his still running the old firmware. Re-did my Ear Tip Fit Test using my AirPods and it now says that it’s not a perfect fit. When I bought it, it said it was.

Anecdotal, but from a source who knows this space. I hope Apple fixes this.