Preprocessor loading works on base hooks provided by hook_preprocess in preprocess_preprocess. However this only provides the base hook, and additional hook_preprocess needs to be called for any Bundles and Views for more specific hooks. This can be improved by including the hook's associated suggestions.
Fixed
1.0
Code
The change is currently missing an automated test that fails when run with the original code, and succeeds when the bug has been fixed.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Regarding #13 ✨ Add Preprocessor Loading for Suggestions Fixed , here is a patch that allows deeper pre-processing: https://www.drupal.org/project/preprocess/issues/3112575#comment-14882622 ✨ Deeper Preprocess Suggestions don't load without a Template or actual hook_preprocess RTBC