This feature (and the bug) was introduced with
STANBOL-1440. because of a equals(..) check with the wrong field of the FstLinkingEngineComponent engine instances registered for unregistered SolrCores are not closed. Because of this also the SolrCore.close() method of the SolrCore used by this engine is not called. This causes those SolrCores (that are no longer registered with any Solr CoreContainer) not to be closed (as the reference count of those core will not reach zero).
However NOTE if a SolrCore is updated the old one is freed by the FST linking engine and closed (as after the switch to the new one the engine automatically frees the old). So this only happens if a SolrCore is removed (e.g. if it is removed from the ManagedSolrServer or if the SolrServer is deactivated as a whole).