This was noticed originally by @DanRoscigno while using the heartbeat setup command to install the default heartbeat dashboard. One of the visualizations is a pie chart with two slices based on filters (defined in Heartbeat-http-monitor.json. This pie chart is broken in 6.5.0, but works in 6.4.0.

When viewing the pie chart/dashboard the following appears in the lower right corner:

There are no errors in the console.

To reproduce, run heartbeat setup, then view the heartbeat dashboard. The affected chart is the pie chart at the top right.