I fixed it by commenting out the following in /etc/logstash/logstash.yml and restarting logstash:
path.data: /var/lib/logstash
path.logs: /var/log/logstash
xpack.monitoring.enabled: true
xpack.monitoring.elasticsearch.username: logstash_system
xpack.monitoring.elasticsearch.password: password
#xpack.monitoring.elasticsearch.hosts: ["https://10.XX.XX.222:9200"]
Logstash now appears in Stack Monitoring on Kibana.
Thank you for the support Badger. Your last entry clued me into what could be the problem.