Initially I filed an issue over at Drupal Exchange because I had no idea why I HOOK_preprocess_node() has not been triggered.
By now I think it is an DS issue because when I do not select a DS layout for my content type HOOK_preprocess_node() gets triggered. But when I select a layout, HOOK_preprocess_node() does not anymore get triggered.
Closed: works as designed
2.3
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I was able to trigger it using hook_preprocess_ds_entity_view:
function mytheme_preprocess_ds_entity_view(&$variables) {
$variables['node'] = $variables['content']['#node'];
$variables['view_mode'] = $variables['content']['#view_mode'];
mytheme_preprocess_node($variables);
}