I've seen some really sporadic posts about this issue all over the internet.
I figured this was due to having my AirPods Pro serviced recently and after contacting apple that's exactly the case. Apparently this is an indicator of the unit being serviced at some point. I'm not sure why apple don't just embed a new serial number for replacement AirPods and link them to the serial number of the original AirPods Pro case (if they didn't replace the case).
This is an AirPods Pro specific "issue" and doesn't happen for AirPods 1 or 2 or with the wireless charging case.