• nvim --version
  NVIM v0.3.0-1295-g5b2cee03c
• Vim (version: 8.1) behaves differently?
  no
• Operating system/version:
  Void linux
• Terminal name/version:
  termite
• $TERM:
  xterm-termite

Steps to reproduce using nvim -u NORC

With file contents

abc abdef
| <- cursor here

Typing ab<C-N><BS>d in insert mode gives 1 completion item: "abdef".
Calling nvim_input('iab<C-N><BS>d') (or an equivalent feedkeys() call) gives 2 completion items: "abd" and "abdef".

Expected behaviour

I would guess that the word being completed should not be in the completion list, but in any case nvim_input() and actually typing the keys should behave the same.