In my form, I programmically make the fields mandatory. The form also has a signature field which throws a validation error on the required fields, only when the form is closed. Ideally, there should be an error when the form is signed.
Paul replied - "I ahve duplicated the issue and found that if I do not lock the fields with a collection of objects then the problem does not occur. By the way you shodul not include the signature field in that collection. It looks like it is a bug and you shodul take it up with support. There may already be a fix for this but I am not aware of one."