To reproduce:

$ vim -Nu NONE some_file

:vimgrep  /./ %
:vertical leftabove copen

I expected the quickfix window to be opened on the left, but instead Vim opens it on the right.
Without :vertical, the issue persists:

:leftabove copen

Opens the quickfix window below the current one, while I expected Vim to open it above.

According to :h :lefta:

Doesn't work for |:execute| and |:normal|.

So, I thought that maybe :leftabove doesn't work with :copen, like for :execute and :normal.
But this remark is also present in :h :topleft and :h :botright, which doesn't prevent these modifiers to work with :copen:

:bo copen

Correctly opens the quickfix window at the very bottom of the screen.

:to copen

Correctly opens the quickfix window at the very top of the screen.

The same issue applies for the quickfix commands lopen, cwindow and lwindow, as well as the modifier :rightbelow, and the synonyms :aboveleft, :belowright.

I searched on the issue tracker (is:issue leftabove, is:issue aboveleft, is:issue belowright, is:issue rightbelow) but couldn't find anything which seemed relevant.

Environment:

• vim --version: VIM - Vi IMproved 8.0 Included patches: 1-1400
• Operating system: Ubuntu 16.04.3 LTS
• Terminal emulator: rxvt-unicode v9.22
• Terminal multiplexer: tmux 2.3
• $TERM: screen-256color
• Shell: zsh 5.3.1