Why doesn't drush check for updates of dev versions of my modules installed by composer?
Drush relies on information written by the Drupal.org package manager to determine whether or not updates are needed to modules or core.

If you use git to install Drupal modules, then you may also use the git-deploy module, and it will add the information needed for Drush to do the update.  Git deploy also works for modules installed via Composer using git; Composer uses git to install when using the --prefer-source directive, or when the module version installed is a development version.

If you use the Composer option --prefer-dist, then Composer will install the tarball packaged by drupal.org, which is why git-deploy is not needed in most cases for stable modules.

Note that there is a bug in git-deploy that prevents it from working when the path to the Drupal root contains spaces. There is a patch to fix this problem available in the git-deploy issue queue; see #2672166.