Summary

Changes to the Due date field are not reflected on the basic roadmap timeline in Jira.

Check for duplicated "due date" custom field

Jira's roadmap timeline only supports values in system default fields. 

If you happen to be using a custom field with the name "Due date," any changes made to this custom field will not get reflected onto the Roadmaps of Jira.

Verify if there is an additional field called "Due Date":

1. Navigate to Settings > Issues

2. Under FIELDS, select Custom fields

3. Search for Due Date and see if more than one field is returned
   1. Note that the system field will not be shown in the Custom fields section
4. If so, rename any duplicates to distinguish them elsewhere in Jira

Now that any duplicates have been renamed, check the issue screens in any affected projects to see if those fields are present.

1. Navigate to your affected project and select Project Settings
2. Select Issues > Screens
3. Select the relevant screen scheme and edit any of the relevant screens
4. Replace the duplicated fields with the system default Due Date field