I tried reloading a save multiple times, so it wasn't just a one-off bug.
It's the scene right after you sleep with Mizora, when he finds you the morning after. At first he's there, but as soon as Mizora says her line about Halsin's biceps being bigger, Halsin goes invisible and the rest of the scene plays out with Halsin's disembodied voice coming from the scenery.
