We have several forms which contain a custom button to submit and save the InfoPath form into the SharePoint library. The default submit and save options are turned off to make sure it runs the rules in the custom submit button.
I noticed a rather strange bug the other day. Actually, several users already reported to me on different forms that the submit button in particular forms are gone. I had no explanation at that time but to have them process a new form.
Until it happened to myself. I canceled filling out a form and a little InfoPath window popped up and asked if I’m sure to cancel or if I would rather like to submit it.
I did click the submit (not the button in the form, but the button in the window) It then submitted the form, threw me an error, and the submit button in the form was gone, vanished, no longer there.
So if you ever run into the same problem that a user managed to have a form without your custom submit button, there obviously seems to be a bug?!