Interface graph issue datapoints always shown as connected instead of null

GRAFANA version : 7.3.7
ZABBIX version : 5.0.4

We are facing an issue with interface graph coming from ZABBIX. When device is unreachable, GRAFANA is getting from ZABBIX and is supposed to show blank area in interface graph as we have set “Stacking and null value” to null.
Unfortunately this doesn’t happen, we saw datapoints as “connected” on interface graph. On ZABBIX graphs, we see empty area (what we expect).
Did we miss something in configuration ? Are we facing a bug ?

Your help will be much appreciated.