I don't know for sure how to reenact this. But I had a stack of over 10k construction drones and another 60k in the works at the time. Every time I upgraded a building it wouldn't move from the parking spot and just ignored the upgrade. I found out I have to manually move them all closer. But, thats such a bad feature if its not a bug.
Is it a large map size? I think a long time ago I made it so that bots only came to build if within a certain radius, to improve pathfinding performance. That was before stacking was available. I suppose with stacks, those limitations might not be needed anymore...
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