I ran another test that confirms that this is hardcoded behavior tied to the 60% archer threshold. I composed a flank of 58% archers and 42% light infantry. I modded their defense values such that the light infantry would die at a higher rate, allowing me to observe the tactics of the enemy flank before and after the 60% archer threshold. Under 60%, the enemy flank tactics behaved normally, but as soon as the fraction of archers exceeded 60%, the enemy flank's tactics changed daily. They fired Charge on Undefended as soon as it became available (interrupting their existing tactic) and thereafter chose a new melee tactic each day. But this behavior happened even when I commented out the Charge on Undefended tactic. As soon as my flank hit 60% archers, the enemy flank's tactic was interrupted and they chose a new skirmish tactic every day. So it does seem to be an ill-executed attempt to allow Charge on Undefended to fire as soon as the opposing flank hits 60% archers, with the problem being that the hardcoded tactic interruption does not cease after Charge on Undefended has fired. But I think it's worth pointing out that the hardcoded tactic interruption should not just be "fixed" but removed altogether because it frustrates modding, and the same effect (getting Charge on Undefended to fire before 12 or 18 days of battle have elapsed) could be achieved by instead changing the scripting in 00_combat_tactics.txt, e.g. by decreasing the duration of skirmish tactics.
