Do you have any proof of this, especially if it's happening multiple times.
It is impossible for potential to change beyond green->blue->black->orange->red
So if it indeed was happening it would be a bug.
What I'm guessing happening is you get a recruit with 20 green ath and after the first practice he is still 20 ath but now blue potential, which is not a bug. The ath rating is actually a decimal they just round it up so the green ath might be 20.21 which shows 20 and the blue ath is 20.44 which is still twenty it just means the cap rating of the player is 48 with some decimal.