It should be caused by carelessness.
\Drupal\Core\Theme\Registry::postProcessExtension
the line 721:
if (isset($cache[$hook]['preprocess functions']) && !in_array($hook, $cache[$hook]['preprocess functions'])) {
It should be:
if (isset($cache[$hook]['preprocess functions']) && !in_array($preprocessor, $cache[$hook]['preprocess functions']))