Leviathan has two sides to attack. The head has front/flank, the tail has flank/rear. Cloud of Darkness (if I remember correctly) has no facing and treats all attacks as hitting proper positionals. This was a fix they implemented early on to fix issues Melee dps had with fights like Demon Wall in Amdapor Keep. I don't believe the game currently has any fights where positionals are simply impossible to get.