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.