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Okay, I figured it out myself. In this example, the game difficulty is set to easy (80%). The AI behavior is different! I ran a test on all other difficulty levels, and the AI behaved as in the Ichthyander example.
For example, if you take Luna (expert fire, Castle or Fortress), and try to capture the Griffin Conservatory (the minimum with a squad of Royal Griffins) on easy (80%) or normal (100%) difficulty, the AI will also behave differently, and you won't be able to capture the Conservatory. The Royal Griffins won't wait until round 3.
https://www.youtube.com/watch?v=5vjnyHIV3ho
I can send you a savegame without any problems, but it will be easier for you to check it yourself on easy (80%).