Here is a recipe for reproducing the bug in emacs-26.0.90 -Q:

Create a directory /tmp/test-copyright
Copy the attached file_1.c-save to that directory
Start emacs -Q

(require 'copyright)
(add-hook 'before-save-hook 'copyright-update)
(find-file "/tmp/test-copyright/file_1.c")

C-x 2
C-x o
C-end
;; insert '// changed line'
C-x b *Messages*
C-x o
C-x b *scratch*
C-x o
M-x save-some-buffers
!
y
C-x b file_1.c

This shows the bug: ', 2018' was inserted on the last line in file_1.c,
not the copyright line.

The fix in the patch given above is "include 'switch-to-buffer' in the
'save-excursion'"; it is not sufficient for this test case, although I
assume it is sufficient in other cases.

A discussion on emacs-devel starting at
https://lists.gnu.org/archive/html/emacs-devel/2018-01/msg00024.html
includes the suggestion to delete 'save-window-excursion'; that does not
fix this test case, and in addition it causes file_1.c to be not
displayed while asking about updating the copyright.

The root cause of the bug is that emacs saves a point for each buffer in
each window; when file_1.c is displayed in the lower window, point is
restored to where it was last displayed in that window; on the last
line. 

The attached patch copyright.diff fixes this test case in a minimal way,
by saving the copyright-end point, and restoring it after displaying the
buffer. It also includes 'switch-to-buffer' in the 'save-exursion' to
address the other use cases.

The email thread mentioned above also discusses trying to preserve
frames as well, since switch-to-buffer can pop up a new frame with
certain user settings. This patch does not address that, since it is not
a regression in emacs 26. A more thorough redesign of this function, to
be cleaner and address restoring frames, is left for emacs 27.

-- 
-- Stephe