I attacked a city with too many defenders, including their sovereign.  That sovereign was not on the battlefied.  I used Karavox's Silver Tongue to "convince" a unit of warg riders to join my cause.  When the battle was done, I controlled the city, but my army was still outside (in the tile I started in).  The unit I bought was also in that tile, and in fact filled up the last part of my army (though he also showed up as a seperate army, kinda).  The sovereign was over the city - not garrisoned, but just sitting on the tile.

Opening the Army Management window to get rid of the second army gave a duplicate view of each army:

Here we can see the Warg Riders with a unit count of two in the lower right:

And here they show as being the leaders of the same army:

Switching between the two images is done with the arrows next to the Army button that brings up the Army Management screen, so it seems it really believes that there are two armies there.

Forcing the Warg Riders out of the first army (by focusing on them in the main army, then clicking the arrow) pushed them to a nearby tile and "killed" the second army.