Kibana TSVB shows keyword string "/" as "(empty)"

[hylowaker (JaeHeung Han)]

I loaded Metricbeat dashboards into Kibana.

The Disk Usage [Metricbeat System] ECS library shows Top 10 mount points with high disk usages.

I see an (empty) mount point, but the actual value of system.filesystem.mount_point field is /. Why is it displayed as empty?

Kibana version: 9.1.2
Metricbeat version: 9.2.4