1. Run 'emacs -Q'
2. Open a new Org file 'C-x C-f /tmp/test.org'
3. Insert the following contents:

* TODO this is a test                                                   :tag:
* TODO here is another test                                             :tag:

The tags should align to the right with the standard monospaced font.

4. Run 'M-x variable-pitch-mode'.
5. The tags are now misaligned.

Please see attached screenshots for the before and after states of the
'variable-pitch-mode' activation.

Running 'M-x describe-char' in the space between the heading's title and
the tag informs us that the space's face is the same as the heading's
(e.g. 'org-level-1').

I am also relaying the technical insight of Adam Spiers.  Please see
attached plain text document or visit the comment's URL:
https://gitlab.com/protesilaos/modus-themes/-/issues/85#note_407147422

Overview of my setup:

+ GNU Emacs 27.1 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.24.22,
  cairo version 1.17.3) of 2020-08-14

+ Org mode version 9.3 (release_9.3 @ /usr/share/emacs/27.1/lisp/org/)

* * *

This message was also posted on debbugs.gnu.org, bug#43272.

-- 
Protesilaos Stavrou
protesilaos.com