Problem with isFieldSet() method

I am facing the following problem with isFieldSet() method on the Sequence of Structure fields.

Consider a field a[0].b .
where a is seq of structure and b is the field I want to access.

When I try to access the field b directly(i.e when I have extracted the seq of structure as a separate event) and use isFieldSet() , even if the field is not set in the Event, it returns true.(i.e gives wrong result)

But when I access the field b using a[0].b, the isFieldSet() method returns false if the field is not set.(i.e gives right result)