This is quite easy to reproduce.
Enable jmx reporting by editing the server/solr/solr.xml and adding the following under the "<solr>" tag:
Start solr with:
Open jconsole and inspect mbeans for solr nodes running on port 8983 and 7574. You'll find that all mbeans (node, jvm, jetty, solr) are present for the solr on port 8983 but completely absent for the solr node running on port 7574.
Same behavior is on 6.6.2 and 6.6.6. However, Solr 7.x and 8.x seem to be fine.