Weird Behavior - GmailApp.getDraft(did).getMessage().getPlainBody();

[Preet Mehta]

So, 

I have created a draft using...

var draftID = GmailApp.createDraft(

    email, 

    subject, 

    emailString,

  { name: "some name", htmlBody: htmlMessage} );

If I use 

.getPlainBody() on a draft got using the getDraft("draft ID")

and then its plain body, using .getPlainBody()

The output varies between following 2 types...

1. Some times it gives emailString, the 3 parameter from the createDraft API, which I believe is the message for non-html based emails.

2. Some times it gives the plainBody extracted from the htmlMessage!

Now, actually I am not able to understand on what basis the output changes?

Can anyone please help?