Assigning variable of derived type - conformant code or not

With the following code 

module m_1
    implicit none

    type t_1
        integer i
    contains
        procedure, private :: t1_ass_i
        generic   :: assignment(=) => t1_ass_i
    end type

    contains

        elemental subroutine t1_ass_i( var, expr )
            class(t_1),       intent(out) :: var
            integer, intent(in)  :: expr
            var%i = expr
        end subroutine
end module

module m2
    use m_1
    implicit none

    type :: T_2
        LOGICAL :: a_flag
    end type
    
    type:: T_3
        type(t_1) :: a_date
    end type
    
    type :: T_4
    end type
    
    type, public::  T_5
        type(T_2) :: v_t2
        type(T_3) :: v_t3
        type(T_4) :: v_t4
    end type
end module

program test
    use m2
    implicit none

    type(T_5) :: a1, a2
    a1%v_t2%a_flag = .true.
    a2 = a1
    print *, a1%v_t2%a_flag
    print *, a2%v_t2%a_flag
end program test

the result I expect is:

T
T

but we get:

T
F

the fact that commenting the line 

        type(T_4) :: v_t4

leads to the expected result makes me think this is a compiler bug, but I am not 100% of my interpretation of the standard re components with defined assignment.

I am currently using ifort v17.0.4.210 Build 20170411