The chart "Available Downtime before SST Required" has to be fixed on PXC_Galera_Node_Summary dashboard. Formula has to consider using metrics from job with different scraping interval.
mysql_global_status_wsrep_* is collected with job "mysqld_exporter_agent_id_xxxx_hr-5s"
mysql_galera_gcache_size_bytes with job "mysqld_exporter_agent_id_xxxx_lr-1m0s"
The fix is provided by Francisco Miguel Biete