So here is my story (sorry it is long, hoping it is helpful):
I noticed the my mid 2020 APP had a crackling sound from the microphones whenever there was a loud sound in noise cancelling and transparency modes. I also noticed the bass starting to be lacking and always failing the fit test. My APP look brand new, I wipe them off with my shirt after using and use blue tack to clean the grills occasionally.
I called apple support and since I have AppleCare+ I was able to get express replacement. The revised buds showed up today. The left (0C6K, FW: 4A400) and the right (0C6J, FW: 4C165). Both are rather new, with the left assembled in January 2022 and the right in February 2022. The inner on both new ones are much more recessed and they actually catch the fabric on my shirt when wiping them.
With the firmware mismatching I was having trouble getting the left to update since the right on is already up to date. I tried pairing the left only to iOS, leaving in the case to see if it would update on its own. It didn’t. Then I found this link and I guess when doing these steps it made the right one update the left one. But it was quick and worked perfectly.
https://www.reddit.com/r/airpods/comments/fyv9ep
The new buds have much more bass and no more crackling. They also pass the fit test now. I will try noise cancelling with the leaf blower tomorrow, but I suspect it will be better too. If you can get your replaced with the revised version, I suggest it.