How did you install neovim? If you installed from source, double check that you followed the instructions, i.e. install build dependencies and then run:
make CMAKE_BUILD_TYPE=RelWithDebInfo
sudo make install
Also, double check the version of nvim in your PATH matches:
nvim --version
Oof, that's annoying.
Weird that
:syntax off
doesn't work, from a small test it seems to do the trick for me. But I guess as long as vim works there's no need to replace it 🙂