A panel loses data points while others don't

Hi, all.

I use mongodb_exporter to scrape mongodb metrics to prometheus, which is configured on grafana. But there’s always one panel which indicating QPS, loses some data points while other panels are OK.

This is so strange to me. How should I solve this ?

The Stacking null value is null for all panels, so changing the CRUD panel to null as zero is not right, actually it’s not zero during the test.