It's not a bug, the number of warns wont be reset even after the player is banned. The ban time depends on the count of warns & the player will only get banned every 3 warns( 3, 6, 9, 12, ... ). If the player has 3 warns, he will get 3 days ban, 6 warns = 6 days ban, 9 warns = 9 days ban and so on..