If I am not mistaken stacks with a night fighter general support nearby battles fought at night, no matter if it is an offensive or defensive one.

Yesterday two Mongol stacks both attacked Antioch at night and my supporting army without a night-fighter did not help, but the supporting Mongol general must have had night fighter.

In another thread it is mentioned that a five star general will get night fighter... so every Mongol general should have it except, on their very first turn after appearing. That would explain why it is usually (one of) the last trait(s) in the list...