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microdesc_cache_reload() calls microdescs_add_to_cache() with listed_at == -1, so md->last_listed was defined by annotation loaded from disk only.

  if (listed_at > 0) {
    SMARTLIST_FOREACH(descriptors, microdesc_t *, md,
                      md->last_listed = listed_at);

But if unsigned time_t then last_listed updated to 0xFF..FF value so that md never be purged by microdesc_cache_clean(). If such md was flushed to disk during rebuild cache then it will forever live.