I have experienced the same problem. Got my pair day 1, so had to be the first batch. At first, good seal with medium, but since I use large for other IEMs I switched to large. After a few days, I couldn’t get a seal on the left. I swapped tips, still the same: left failed, right passed.
I cleaned the tips according to the manual, no change.
Then I contacted Apple, told support that the left one doesn’t seal anymore and I experience less bass on the left.
Apple sent me a replacement for the left AirPod. While it was on the way, I repeated the fit test. Suddenly, both failed to seal.
The left replacement arrived. After initial difficulties to pair the new one, because both seemed to have different firmware versions, I put on a new tip in size large and ran the fit test. Result: left (new AirPod, fresh tip) passed, right (old AirPod, old tip) failed.
So then I swapped the tips and ran the test again. The result:
Left (new AirPod + old tip) = Good seal
Right (old AirPod + new tip, which had a good seal and on the other side seconds before) = Fail
I ran the tests multiple times and also tried adjusting them when it failed. The results are consistent.
Since I do hear a difference when the test reports a good seal in form of better bass and ANC performance, I don’t think it’s the test software.
I don’t think that it’s the tips either, at least it’s not the main cause.
My theory is that dirt, sweat, gunk etc. blocks parts of the tiny black vent covers after just a few days of wear and this changes the sound characteristics, which are then picked up by the internal mics differently and therefore don’t match the reference recording. If this is the case, then the sound quality of every AirPods Pro will deteriorate quite quickly.
I don’t see how the grids can be cleaned effectively to prevent this. I have very clean ears, take care of the AirPods and it still happens to them.
Another possibility would be that the microphones are dying very quickly, but I don’t think that this is very likely.