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I should mention that I am unsure if this is a bug, but I think it might be.
Details: I was sitting beside a fort of an enemy as someone attacked the fort. I did not receive an option to reinforce the attackers of the fort. At first, I thought this might be because I was not allied to the attacker, so I made sure to ally with him and we tried again next turn. I was still unable to reinforce his army on the second attempt. The alliance showed me the attacker's territory, so I think this was sufficient to allow for my reinforcing their army?
Again, I am unsure if this is a bug, but I think it might be. I am relatively certain I was in the zone of allowed reinforcement, but I could be wrong. If I am wrong, it seems that it was possible for me to be in the center of the settlement and not be triggered as a reinforcement.
Edit: I was playing Yuan Shua and the attacker was playing Liu Biao. I was in proximity to a settlement and I think it was called Dong, a minor settlement.