deploy returns 'Operation not permitted' with chgrp on shared_dirs

[memen45]

A while after the first deploy, I wanted to update, so did a deploy again. In the mean time, the web user (www-data) added some files to some of the shared_dirs, also making some subdirectories. These files and directories are created with owner www-data:www-data and this should not be a problem.

Now, trying to deploy, the dep deploy returns an extended list of all those files and directories with e.g.
chgrp: changing group of 'pub/media/tmp/design/file': Operation not permitted
The deploy process is ran from the user 'deploy' with the following settings:

set('writable_mode', 'chgrp');
set('writable_recursive', true);
set('http_group', 'www-data');

Effectively, the command that is executed by the deploy user is chgrp -H -R www-data writable_dirs, however, chgrp is not allowed on files or directories that are not owned by the user calling it. Since the new files are created by www-data:www-data, this command fails, even thought the group actually already is www-data. So, the complete deploy fails, while nothing is wrong with the file permissions.

How can this be solved? Is this caused by the overlap of shared_dirs and writable_dirs? I can imagine that permissions do not have to be repeatedly set for shared_dirs, as those are not created again for each deploy. So only the first deploy should set permissions for shared_dirs AND writable_dirs, but subsequent deploys should only set permissions for writable_dirs that are NOT shared_dirs (with one exception, in case the shared_dirs have changed, the new directories have to be created and permissions have te be set).