Type-bound assignment reinitializes the members to zero

Question

I am new to fortran and trying to implement a matrix class in it with the use of dynamic memory allocation and pointers. When I was trying to overload the assignment operator, I observed that the object members get reinitialized to zero on the entry to the assignment subroutine. Following is a simple program that reproduces the problem.

module matModule

    implicit none
    ! Define the data kind for integer and real numbers
    integer, private, parameter :: iKind = selected_int_kind(8), rKind = selected_real_kind(15,307);

    type mat
        integer(kind=iKind), public :: nrows,ncols,npage;
        real(kind=rKind), pointer, public :: vals(:,:,:) => null();
    contains
        procedure, private, pass :: assign_scal_r
        generic :: assignment(=) => assign_scal_r
    end type mat

    contains
        subroutine assign_scal_r(this,value)
            implicit none
            real, intent(in) :: value
            class(mat), intent(out) :: this
            integer(kind=iKind) :: err
            print *, loc(this%nrows), loc(this%ncols), loc(this%npage)
            print *, associated(this%vals), this%nrows, this%ncols, this%npage
            this%nrows = 1; this%ncols = 1; this%npage = 1;
            if ( associated(this%vals) ) then
                print *, "The array on the left is already allocated."
                deallocate(this%vals, stat=err)
                if (err /= 0) print *, "Error deallocating the variable."
            end if
            allocate(this%vals(this%nrows,this%ncols,this%npage), stat=err)
            if (err /= 0) print *, "Error allocating the variable."
            this%vals = value;
        end subroutine
end module matModule

program main
    use matModule
    implicit none
    type(mat) :: k
    k = 5.0;
    k = 1.0;

end program main

The output of this code is

140700383959168       140700383959172       140700383959176
F           0           0           0
140700383959168       140700383959172       140700383959176
F           0           0           0

Note that the output is generated from within the assignment subroutine. The variable 'k' in the main program goes through two assignment operations. It is understandable that the members are by default initialized to zero during the first assignment. However, the members contain 0 when the assignment subroutine gets executed second time. I am also printing the memory addresses of the members which, as per my understanding, show that the values of members get overwritten during the call to the assignment subroutine.

I am not sure if this is expected behavior. If it is an expected behavior, then my question is that does the pointer get deallocated automatically on the entry of the assignment subroutine? I have tried this on both Intel Fortran compiler and gfortran and both produce same results.

Pardon me if this whole problem is caused due to some stupid mistake I made in my code somewhere.