This is what happens: Baron goes into its firing sequence; turns to face target (you, the player) Baron continues going through attack sequence; turns to face target again Lost soul behind baron hits it, doesn't cause it pain or interrupt its firing sequence, but does cause it to change target Baron reaches end of firing sequence; scratches target (the lost soul) but doesn't turn to face target again That is, the baron hasn't attacked the player at all, it's attacked the lost soul behind it.

The interesting part is that it doesn't turn round at the instant of attack (when it makes the scratch sound). It turns out that barons - and hellknights, they both call the same code pointer, A_BruisAttack - appear to be the only monsters which don't turn to face their target on the instant that they attack. The call to A_FaceTarget is omitted from A_BruisAttack in what I can only assume is an oversight by Doom's authors.

Incidentally, if you put the A_FaceTarget call into A_BruisAttack and play the demo, the baron happily spins round on the spot and smacks the lost soul behind it. The change doesn't affect demo sync, at least in this case.