Symptoms

When user is managing issues with sub-task. Hovering mouse over selected sub-task does not show buttons to change order of sub-tasks.

Expected behaviour 

Buttons to reorder sub-task are visible.

Workaround

Enable drag and drop subtasks feature. WARNING: This feature is in development and may sometimes behave unstable. It will not break your data, but UI may not show exact results of the operation, refreshing the page will fix the glitch.

1. As a JIRA administrator, navigate to <JIRA-URL>/secure/SiteDarkFeatures!default.jspa
2. Add the following dark feature flag: com.atlassian.jira.issuetable.draggable

You may also want to also add com.atlassian.jira.issuetable.move.links.hidden feature flag to hide in UI the place for reorder buttons.