Middle tree Arcstrider (Way of the Current) cannot make use of the Heavy Handed mod with its subclass-specific charged melee attack, Tempest Strike. It seems to be the only one too, so this doesn’t seem intentional at all; even middle tree Gunslinger (Way of a Thousand Cuts) can proc it with its ranged Knife Trick melee attack, so it makes zero sense for middle tree Arcstrider to not be able to as well.