Grafana logs panel cannot highlight search expression

Search Expression highlight works well at Explore with loki datasource, but works wierd at panels with type logs.

It seems that only log lines at bottom will be highlighted with matched words (no matter descending or ascending order), and the number of lines highlighted is not certain(maybe tens or hundreds lines).

So is this a known issue for grafana/loki or a miss configuration? Wait for your help thanks.

loki-stack: helm chart versoin 0.38.1
grafana: grafana/grafana:7.0.0
loki: grafana/loki:1.5.0
promtail: grafana/promtail:1.5.0