I just got mine serviced today through the AirPods Pro Service Program for Sound Issues. Mine was out of warranty but was having issues with static (white-noise like) sound emitting through both sides with ANC turned on as well as Transparency. They also would jump back and forth between OFF and Transparency mode even though I had set ANC on. This set was also already a replacement set back in January. I initiated a request through Chat support, and they did not want to honor a free replacement at all and said they wanted to charge me for a replacement. So I disputed that over the phone and they set up a Genius appointment for me. Had a lovely girl assist me and just ran a diagnostic test through her macbook and within about 5 minutes out came a replacement set on the table.
My replacement set originally showed a serial number of "?", but after connecting them to my phone a resetting them, I plugged them in (inside the charging case) and let them sit for about 10 minutes. The firmware upgraded from 2D27 to 3A283 and the serial number now shows up correctly and aligns with the serial number that shows on the top right hand corner of my Apple Store Work Authorisation and Service Confirmation.