Allies suddenly blackflag during war for no reason

[The Good Shepherd]

I just encountered this error.
War Allies' armies suddenly turned exiled while;
A) attached to my army within my own territory, and
B) besieging an enemy fort.

Needless to say, this had dire consequences for the war, as they sauntered off back to their own provinces I was massively outnumbered.
At no point did either of my war allies change their various military access during the war.
I grabbed some screenshots from steam but I can't seem to find the bloody things.
Quite frustrating, as this playthrough (Manipur achievement circa 1500) requires a restart.

 

[Dominion]

Did they declare a different war?

 

[The Good Shepherd]

Did they declare a different war?

No, they were only fighting in this one war - screenshots to follow

 

[The Good Shepherd]

Here you can see Hsenwi running off in Exile at 2 pivotal moments.
Either this is bug or a "cheat" from the AI.
Both my allies were only in this one war. In almost 2000 hours of play I've never seen this.

 

[Ultima_Ratio]

I've had the same thing happen - and in the same general area (as Taungoo). I think it has something to do with them detaching their army from yours.

 

[Yaguar]

I've seen this in atwix' Taungu achievement run on very hard, and he and his twitch chat sorted out the reason. Check here:

 

[Dominion]

With all due respect, I'm not going to watch more than 7 hours of Atwix for a single gimmick

Timestamp please.

 

[Yaguar]

With all due respect, I'm not going to watch more than 7 hours of Atwix for a single gimmick

Timestamp please.

I think it was in the first war against Pegu, so somewhere between minutes 20 and 30. Gonna check that tomorrow.