"Spirit Blade" Trait bugged/doesn't activate

Despite having put a skill point into Spirit Blade (a Trait in the Antivan Crow section of the Mage skill tree), it has never activated. Instead, Rook just does the normal Heavy Attack. (This is after a Light Attack Chain, against an enemy with an Arcane Bomb debuff ready to be triggered.)

Possibly related: I remapped Heavy Attack from Shift+LMB to MB5.

Edit: Thanks to someone in a Steam thread about the same issue, I realized that it doesn't require an Arcane Bomb to trigger, so I started testing by just attacking nothing. Conclusions:

• I can sometimes get Spirit Blade to trigger when using very specific, precise timing to make the game realize I'm going for a combo.
• Sometimes, even with specific, precise timing, it doesn't trigger.
• Sometimes, even with a deliberate weird pause in the middle of the combo, it does trigger.

Even if this Trait actually worked 100% consistently, it would still be pretty useless, since it requires a long chain to set it up, at a time in the game when a lot of Rooks using it probably have the Arcane Strike Ability Upgrade that makes Strike abilities apply Arcane Bomb. The fact that the trigger seems to be half-busted just makes it even worse.

Spirit Blade should just entirely convert your heavy attack to the ranged version.