Panels below 12 hour time range are blank when using PerSecond function

We have added panels to Grafana 6.7.1 with Graphite as the data source and collectd disk and network stats as the metrics sources to several dashboards.
When the persecond function is applied to the queries on those panels, they are blank unless you set the timerange >=12 hours.
If we remove the persecond function, they work normally.

Has anyone else seen this behavior?
How do we fix it?