Defeated Pretender keeps on fighting...?

[Chazar]

I one of my multiplayer games someone else's nation was almost defeated. There was only a single big army led by its pretender left. The army went sieging its own capital, which was owned by enemies. Now the siege Lasted some turns and meanwhile all other provinces where lost, thus the game send that standard message that the pretender had been truly defeated...

...but the siege continued under the independent's flag and my scouts even report that they see a Nataraja among the sieging force?! Does it make sense for a defeated pretender to stay on?

I do not fear to intervene, but what would happen if I dont? Will the AI take over that independent Pretender an storm the fortress eventually or will it just sit and siege? I mean, I cannot even preach that Nataraja away, so this is really odd...