bug#39067: shell-command-dont-erase-buffer strange behaviour

[Madhu]

C-h v shell-command-dont-erase-buffer
C-h f shell-command

Cut to the chase with the test case:

(let ((shell-command-dont-erase-buffer 'beg-last-out))
  (with-current-buffer (get-buffer-create "OUT")
    (erase-buffer)
    (shell-command "/bin/echo FOO" t)
    (shell-command "/bin/echo FOO" t)))

The result (as expected) is a buffer named OUT with 2 lines FOO.
The same result is expected with the following code:

(let ((shell-command-dont-erase-buffer 'beg-last-out))
  (with-current-buffer (get-buffer-create "OUT")
    (erase-buffer)
    (shell-command "/bin/echo FOO" "OUT")
    (shell-command "/bin/echo FOO" "OUT")))

However in this case (and in some other cases) shell-command erases the
"OUT" buffer despite a non-NIL binding of
shell-command-dont-erase-buffer

(at least since emacs 25.2)