BufNew not triggered for matching file specified on the command line

I have the following in my ~/.vim/filetype.vim:

augroup markdown

    au! BufNew *.md echom "I was called"

augroup END

This gets executed if I launch vim, then execute:

    :e foo.md

However, if I launch vim like so:

    $ vim foo.md

This does not get called - the message is missing from :messages

This behaviour appears to contradict the docs:

http://vimdoc.sourceforge.net/htmldoc/autocmd.html#BufNew

Is this a bug?

Here's my :version

VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Apr  1 2016 13:05:21)

MacOS X (unix) version

Included patches: 1-1655

Compiled by Homebrew

Huge version without GUI.

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