Behavior allows you to change how certain tasks are represented in the chart.
Treat parent issues as groups
A group aggregates values from the tasks it contains, and its own values for manual start date, finish date or work estimation are ignored. You can configure your Gantt chart to show parent issues either as actual tasks or as groups (containers for tasks).
Gantt groups issues by default. To have your parent issues shown as actual tasks, on the Scheduling screen, under Groups, uncheck the "Treat parent issues as groups" checkbox.
If an issue is treated as a group, any resources assigned to that issue will be ignored when calculating resource usage.