https://userapps.support.sap.com/sap/support/knowledge/en/3322931
3322931 - HOST_* views don't collect data on SYSTEMDB but collect data on tenant | SAP Knowledge...
You notice that the HOST_* views are empty on SYSTEMDB but not on tenant. e.g. select * from "_SYS_STATISTICS"."HOST_SERVICE_THREAD_SAMPLES"