That happens because Steam sometimes treats the Switch Pro Controller as if it doesn’t have analog triggers, so it maps both L and ZL (or LT) to the same input. To fix it, go into Steam > Settings > Controller > General Controller Settings and make sure “Switch Pro Configuration Support” is enabled. Then, in Hollow Knight’s controller layout (right-click game > Manage > Controller Layout), you can manually remap L and ZL to different actions.

If that still doesn’t separate them, try switching the controller to DirectInput or XInput mode using a tool like BetterJoy or reWASD. Those make Windows recognize the triggers separately so games can tell the difference.