As per the linked ticket below, Jira Roadmap on projects that are running in software only display epics in the timline where the timeoine is added.

We are working to Company Wide hybrid project for internal teams, the key request is to have timeline and board functions to track work and timelines simply, but with teh current Timeline the function is limited. Research as below shows that it will not display the task dates.

https://community.atlassian.com/t5/Jira-Software-questions/Jira-Roadmap-does-not-show-my-stories-tasks/qaq-p/1146651

The request here is that we have the option to display different types in the timlein and at a minimum display 2 levels down with Epic/Task, as you can in a Business Project (Task/Sub/Task).