I already noticed this issue after getting the first pair in the mail on day 1 (Sep 23rd).
Now, I'm on my 4th unit/pair in less than a month.
1st unit- noticed the issue (vocals moved to far left side and stayed there without moving back to centre after turning on ANC) on the same day it arrived thru mail, went down to Apple Store to get it exchanged for a full new unit.
This first pair the earbuds serial numbers both start with GN
2nd unit- Exact same issues. Got it refunded. This pair of earbuds are GX/GN.
3rd unit- Made an analysis with my wife's pair. Her's doesn't seem to have the issue, so took note of her serial number (Charging case starts with GN, and earbuds start with GW/GX), and went to Apple Store to take the leap of faith and got one new unit of similar batch (GHWxxxxXXX).
Issues still exist but not as bad as first 2 pairs. Forgot to take note of the buds' serial numbers.
4th unit- sent them in again after tolerating the nonsense for a few days (somehow i think Version 5A377 could be the cause of it as when i tested them out of the box, the older version seems ok). They finally had some diagnostics tools and as expected, can't find out what's wrong with them and all tests passed. Had only the buds changed, issue still happens but not as unbearable as first 2 pairs. This new pair of buds are also GW/GX.
Not sure why but i suspect GN earbuds are having more obvious drift. When i turned on ANC, i could hear the vocal drift all the way to the back left of my head.
Anyway these couple of days there's quite a lot of discussions on Reddit, and 9to5mac has already officially reported this (Google 'Airpods pro 2 sound shifting/panning/imbalance'). More tech websites are also reporting the same thing. I think it would help if everyone reports this in the Apple Discussion page. I already started a thread there in hopes to raise awareness.
I got so sick of the sound imbalance as it's constantly happening that I'm turning off both ANC and Transparency altogether. Now we can only wait for firmware fixes to happen.
Also, does anyone feel that the Geniuses in Apple stores are trying to hide that they know of this issue? Just by the sheer number of complaints in online forums, there's no way they are oblivious to what's happening. Just my 2 cents.