A question about deformation gradient in ABAQUS user subroutine VUMAT

I have a question about deformation gradient in VUMAT.

When I use one element, Vumat works well and there is no any problem. Initially, only the diagonal part of deformation gradient is one, the others are zero. However, the problem is when several elements are considered, all the components of deformaiton gradient are equal to one at very beginning.

I just did uniaxial tension test (without any rotation). I know there is a corotational coordinate in ABAQUS, so we need use stretch tensor instead of deformation gradient in VUMAT. But I think this has nothing to do with corotational coordinate.

Does anybody know how this happened? Or does Umat do the same thing? Thank you