C-k is bound to
org-kill-line by default (try with
emacs -Q), not to
org-backward-heading-same-level (which is bound to
C-c C-b by default).
One possibility is that somewhere in your init file, you rebind
org-backward-heading-same-level but that has nothing to do with loading
org, so the
eval-after-load may trigger early, but you rebind it afterwards. You should check your init file(s) for such a setting and just expunge it.
[Feel free to stop reading here and try to fix your problem. Afterwards and if you are curious, read the following, although I don't think it has anything to do with your immediate problem; but it may be useful in various obscure situations in the future.]
There is an additional wrinkle with
C-k in particular however, that might be worth knowing (although it's firmly in the advanced category).
C-k is normally bound to
kill-line in most modes. Org mode redefines it but not by setting
C-k in the keymap, but by remapping the
kill-line command to
org-kill-line - see org-keys.el:
You can undefine such remappings, but you have to use something like the following:
(define-key org-mode-map [remap kill-line] nil)
although you should think through the implications.
And even after you do this,
C-k is still bound in the global map to
kill-line, so if you really want to make it do nothing, you will need to undefine it there as well. That is something I would NOT recommend however.
As I mentioned above, this is probably NOT the problem you are having; it is something to keep in the back of your mind for future reference. The moral of the story is that keymap handling can be a complicated subject in Emacs.