I personally have no good idea, but in the old thread I linked, the final solution was to actually enable metrics via the foreman-installer answers file.
This seems counter-intuitive to me, but maybe Puppetserver requires them under certain circumstances? I just checked and it looks like we have metrics “somewhat” enabled. At least we have a metrics.conf and jmx metrics are enabled. In the installer answers file we have:
server_puppetserver_metrics: false
server_metrics_jmx_enable: true
server_metrics_graphite_enable: false
Maybe this helps.