Sorry for the delay in posting a reply but I have been researching your issue. I have been able to duplicate it. In doing my research I did find that if I programmatically force a validation then the desired error message is displayed. So while I am tracking down engineers to see why they do it this way I can suggest a work around.
If you make your actual submit button invisible, then put a second normal button on the form (making sure it is visible). This is the button that the user will see and use for submission. On the click event of the button, you will want to force the firing of the validate events and if they come back false then cause the submit to happen. Your javascript code will look something like this:
if (TextFieldName.rawValue == "" || TextFieldName.rawValue == null){
form1.execValidate();
} else {
SubmitButtonName.execEvent("click");
}
Once I hear back from engineering I will post the result.