Attach related record trigger removes relationship if formul

For triggers of type "Attach related record", if the formula does not return a value - the trigger will return -1 which erases the relationship that already existed. See sample below: I want this trigger to attach a record ONLY if there is not one already attached. However, the trigger is removing the relationship that already existed because rollbase is thinking the formula returns -1 (even though nothing was returned). I will convert this to an object script trigger for now, but the bug still exists. Thanks