I noticed that the AI often gets stuck on large cliffs and lakes. I've seen this on a couple saves but it was never too big of a deal. It is kind of game breaking on my latest game because there is only one path into the base and the biters get stuck on the peninsulas leading up to it. There are always hundreds of biters in the two circled regions and they never attack with more than a 10 biters at time. I can trick them into attacking the base by guiding them around the river.
It looks like the AI is pathfinding using waypoints that are too close together. So it can get stuck on a local minimum and never find a way out. This is probably really hard to fix because it's probably inherent to the pheromone cloud AI algorithm. Maybe you could add a penalty to areas whenever they fail to find a path to/from a location to help them get unstuck?
Screenshot: https://prnt.sc/qamfvh
Save: https://drive.google.com/open?id=1joldwF8EOoVKU_XFwLlm3aVHAa3qEmGL
Mods: Automatic Train Painter 3.2
Base mod 0.17.79
Cargo Ships 0.0.48
Choke Point 1.17.12
Dragon Industries - Nauvis Division 1.17.24
Factorio Standard Library 1.3.0
Rampant 0.17.28
Rampant Arsenal 0.17.18
Resource Spawner Overhaul 5.0.24