The problem:
The M&K object is not recognizing a manual "Click" control that was set up in the App Properties to use the Left Mouse Button as the control key. It will register that the "On Control Pressed" event triggers if you manually set the control state with the M&K object in an action, but will not trigger the "On Control Pressed" trigger if you manually click the Left Mouse Button.
What should happen:
The "On Control Pressed" trigger should trigger both ways: by manually setting the control state with an action AND by manually activating the control with a press of the LMB.
How to recreate:
Open the attached file and run it. Follow the instructions therein and be sure to read the comments in the event sheet. Note that setting the control to trigger through actions works properly, but the trigger will not trigger if you manually click the LMB.
Click control bug example
In the custom controls it says "Left Mouse button" by default when you create a new control, but it doesn't seem to actually assign that key to the control. However it works if you hit the dropdown and manually click "Left Mouse button".