the modification should read
! Membrane forces
norm(1) = (dd(1,1)*eps(1) + dd(1,2)*eps(2) + dd(1,4)*eps(3))*thk
norm(2) = (dd(2,1)*eps(1) + dd(2,2)*eps(2) + dd(2,4)*eps(3))*thk
norm(3) = (dd(4,1)*eps(1) + dd(4,2)*eps(2) + dd(4,4)*eps(3))*thk
! Modify in-plane forces by constant temperature change
norm(1:2) = norm(1:2) - alp(1:2)*d(9)
Looking at this, seems to suggest that there is a missint "thk" on the added term. A patch test with a thickness not equal to unity (1) should reveal the possible error.