The article "Agni Kai" has this sentence in the tradition section: 'In a proper fire duel, each combatant, if male, fights with a bare torso.'
Why doesn't Zuko remove his shirt and fight Azula with a bare torso, like he did when facing Admiral Zhao? This is a very proper fire duel, since it is for the crown of Fire Lord.
Most of the other traditions are followed, except for the gong, such as the siblings facing each other at an open area and at what seems to be around sunset; and Azula takes off her shoulder garment, but of course keeps on her whole robe since she is female. However, although Zuko only has his clothing and no shoulder garment since he joined Team Avatar, does not even take off his shirt to fight with a bare torso.
Did Zuko just not bother to follow this tradition of a bare torso in the fight with his sister, unlike he had in his duel with Admiral Zhao?