CA say this is the AI breaks sieges on loading a game because on load it does a periodic assessment of its position, and decides its forces are better used elsewhere. This implies that the periodic assessment is a more thorough/better (and lengthy?) analysis of the AI's position, compared to the normal end turn.

Dedicated testing by others has shown than if you reload every turn, the AI never expands beyond its initial settlements.

Imagine if the periodic assessment DID occur every end turn - the AI would never expand.

So, we do not have a bug in the save game file, but we now a design flaw (bug?) in the periodic assessment by the AI, which gives worse results than if the AI just did its normal end-turn assessment.