I have looked into eshell and make-term source and narrowed down the

range of the issue somewhat.

If I evaluate the test code from eshell buffer, process-environment

lose "TEST=1234" inside make-term function as below.

(defun make-term (name program &optional startfile &rest switches)

  (let ((buffer (get-buffer-create (concat "*" name "*"))))

    (cond ((not (term-check-proc buffer))

    ;; Here, process-environemnt still have "TEST=1234"

   (with-current-buffer buffer

       ;; Here, process-environemnt lost "TEST=1234"

     (term-mode))

   (term-exec buffer name program startfile switches)))

    buffer))

I still don't know why but process-environment lost some values when

creating new buffer with 'get-buffer-create' function.

You can test this as below:

1-1) emacs -Q

1-2) evaluate below code.

     (let ((process-environment (cons "TEST=1234" process-environment)))

       (get-buffer-create "test-buffer")

       (switch-to-buffer "test-buffer")

       (member "TEST=1234" process-environment))

1-3) you can see "TEST=1234" is a member of process-environment in new buffer.

2-1) emacs -Q

2-2) M-x eshell

2-3) evaluate below code in eshell buffer.

     (let ((process-environment (cons "TEST=1234" process-environment)))

       (get-buffer-create "test-buffer")

       (switch-to-buffer "test-buffer")

       (member "TEST=1234" process-environment))

2-4) you can see "TEST=1234" is lost from process-environment.

You can also test this simply as below. 

3-1) emacs -Q

3-2) M-x eshell

3-3) evaluate below code in eshell buffer.

     (let ((process-environment (cons "TEST=1234" process-environment)))

       (with-temp-buffer

         (member "TEST=1234" process-environment)))

3-4) you can see "TEST=1234" is lost from process-environment.