They're not penetration rounds, so will likely ricochet off the individual. After which, they'll co-mingle with the rounds scattered around by all of the officers. Except in isolated cases that don't involve multiple officers, but then the serial number is a moot point.

So while a serial number can prove that a particular round came from a specific officer, proving a direct connection between any specific damage and a specific round would be an area of uncertainty and conjecture.

And even if you do keep track of an individual round, the above argument itself would allow for officers/lawyers to argue that case anyway, introducing enough reasonable doubt to weasel out of repercussions.