Arcane Victory working only on secondaries?

[Power-Roll]

I've recently been able to max my arcane victory thinking the chances were just too low for activation.

However, on further testing I've realized that the arcane procs 0% of the time with primary and melee weapons.

The arcane only procs with my secondary weapon.

Is this a bug? The description of the arcane mentions nothing of being a secondary exclusive arcane.