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There is no such mod and this is not the only instance of AI inconsistency. They can take over an island you just conquered without buying back shares or attacking it. Luckily it does not happen too often.
Curious bug, we haven't seen something like this before.
Got to ask, though: Are you using any mods, even if there's no specific one that would allow such behaviour?
The AI should generally still be restricted to the same buildings as the players. Did you (or anyone else) encounter something similar before? (or with other buildings?)
If you'd let us know about these points, that would be great.