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Title is self explanatory. When enemy units rout, they shouldn't be able to pass cleanly through your line. Certain defensive battles become almost impossible as attacking units charge, rout, continue forward, and reform behind the line. Now I'm taking fire from two directions, and my line collapses. Routing units that encounter enemy troops should either be killed or captured at high rates. No more 1500+ brigades reforming in the rear.
It shouldn't be a thing, but unfortunately the AI sometimes believes charging into the enemy to get behind them even though they are already routed, is the best way forward.

The J&P rebalance mod helps this to a point, but it'll still happen randomly.