I had all CoS folders installed, straight install into the mods folder only moving the tactical maps.
World Size: Huge
Map Type: Random
Wildland Frequency: Balanced
Production Pace: Default
Quest Frequency: Balanced
World Difficulty: Normal
Monster Frequency: Moderate
Magic Strength: Moderate
Resource Frequency: Moderate
Random Event Frequency: Default
Number of Opponents: 9
Empire vs Kingdom Balance: Even
Typical Intelligence of Opponents: Normal
Opponent Surrender Threshold: Normal
Only conquest victory type.
No customized opponents or sovereigns.
Happened with shadows/centaurs/frost giants
As I mentioned once I removed the rivermod folder, it worked fine, got 5 in a row with it working perfectly.
Also this was an old installation from steam, I had never used mods before, but never uninstalled it after first installing it when LE came out if that makes a difference.
Also I have none of the DLC packs.