the problem OP has is that after a shattered reatreat, buggy battles can happen. In such buggy batles no casualties are inflicted on either side, and the battle rages on and on. then the army routs again, and likely gets tick of morale and reinforcement due to montly tick, leading to unnecessary ping pong before the desired wipe happens.
Its a regular 'bug', I have seen it happen a lot. I'm assuming it has something to do with high professionalism and units in reserve that don't get flagged as below 0,5% morale when shattered retreat happens.
@Sharples88 that was problem here right?