[Agrajag]

Mystery Solved?
This is 100% a bug.
I'm guessing this is because the Wraith Centurion is a summoned immortal.

How do I know?
Check out this 2nd save, I attacked the province where the centurion died, and if you watch the replay you can see that the same centurion has joined the independants and is fighting against me :O

Another guess would be that Illwinter tried to do something like make immortals revive in their home province. The Wraith Centurion has no home province because he was summoned, so he thinks his him is with the independants he died fighting and respawns there (and joins their side).

This really puts EA Ermor at a disadvantage, since summoned immortals is one of the best weapons they have.