Hi @eMitch -- I would add I'm seeing the same issue after upgrading 7.17 to 8.8. I detailed my problems here Stack Monitoring with Metricbeat 8 -- No Cluster Found but just stumbled upon this post now.
From my debugging it appears that the format of the monitoring documents Kibana is expecting, does not match with what metricbeat is sending (See attached post)
I had a fleeting thought that perhaps the intention was to have an ingest pipeline to convert to the correct format (as this has been done in the past on previous versions, e.g. xpack_monitoring_6), but I can't find anything to suggest this is the case.