I think he's suposed to lose curse bearer when you pick the dragonsoul up. That's what it's been like for me.
And as for the bug, it might be that Aequitas didn't do what you did exactly, and that is, zot the dragon, kill the shade, ick the dragonsoul up, and then, if the game spawns a new dragon he'd be unzotted.
That MIGHT be the case.
I almost got pwned by Shifty Brickwork!