Why does this custom validation function not fire

[jetbuster]

Hey there guys,
I have a Managed Metadata control and would like to attach a custom validation function to it.

I have the following test function:

function validateMM(source, args) {   
    console.log("validation", source, args);
    args.IsValid = false;
}‍‍‍‍

and have it attached to the control

When I submit or pause the form nothing fires.
What am I doing incorrectly?