I believe this is part of a more general problem caused by the introduction of a third AI.
Here is further proof from my Campaign as Siberian White: I got some Don Cossack visitors. Lovely guests: they eat up my food and blow my railways up!

leure:
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Whenever troops are in territory belonging to another Faction they start to tear up railways. Is there any line in the code that tells them not to destroy railways if a region is 100% controlled by an allied faction?
Edit: I played this campaign for a further two turns: my guests became polite again and repaired that very same railway themselves (I guess the AI found a new way to waste ressources).
