Provocating, I been wanting to fix this also and been thinking about it but have not made any test runs yet. The only backwoods way I can think of to resolve it easy is to make a double leader situation. LeaderA = invis non agro npc, LeaderB = follows leaderA, Formation adds follow LeaderA also. The formation loc will have to be adjusted tho to make it still look correct. But this way if the leader dies, the formation adds will still follow the invis npc thus simulation the patrol. Then you can also add a depop timer or another for of check on the invis npc controller to work with a depop cycle.
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