What is wrong with this code?

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I’ve modified the code a bit, such that the result variable (b) and the intent(out)-variable (err) in fun1() are always defined, and also initialized err before calling fun1() optionally (the line marked by *):

program main
  use iso_fortran_env, only: dp => real64
  implicit none
  call test_fun1()

contains

  function fun1(a, err) result(b)
    real(dp), intent(in) :: a
    character(:), allocatable, intent(out) :: err
    real(dp) :: b(2)

    if (a > 2) then
      b(:) = -1
      err = "a bigger than 2"
      return
    endif

    b(:) = 1
    err = "OK"
  end function

  subroutine test_fun1()
    real(dp) :: a, b(2)
    character(:), allocatable :: err

    !! err = "dummy"   !! (*)

    a = 3
    b = fun1(a, err)

    if (allocated(err)) then
      print *, 'err is allocated'
      print *, 'next line tries to print err'
      print *, len(err), len_trim(err)
      print *, err
    endif

    print *, b
  end subroutine

end program

Then, if the line (*) is commented out, using Gfortran12 + Ubuntu22,

$ gfortran test.f90 && ./a.out
 err is allocated
 next line tries to print err
           0           0
 
  -1.0000000000000000       -1.0000000000000000

while if the line (*) is uncommented,

 err is allocated
 next line tries to print err
           5           5
 a big
  -1.0000000000000000       -1.0000000000000000

The result also changes on an old MacMini, or with -fsanitize=address. It seems like the length of err is “retained” before and after the call (so 5 chars are printed when “dummy” is set). If err is not defined before the call, some garbage value will be “retained”, so resulting an undefined behavior (-> accessing invalid memory if the random value is large or negative)?

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