The ‘Issue Links’ section (includes ‘relates to’ and ‘is duplicated by’) only appears on the logged out view (even when logged in), corresponding to the old url. ‘Issue Links’ are not present on report.bugs.mojang.com pages. This issue relates to [WEB-8364] and [WEB-8365], where the same pattern appears.
Example
Issue Links are visible on the legacy site:
https://bugs-legacy.mojang.com/browse/MC-2836
Issue Links are visible on the url of the old site:
https://bugs.mojang.com/browse/MC/issues/MC-2836
Issue Links are not present on the view requiring being logged in:
https://report.bugs.mojang.com/servicedesk/customer/portal/2/MC-2836
Issue links are visible via this panel
This is the only way to do it currently as far as I am aware: https://community.atlassian.com/forums/Jira-Service-Management/I-want-to-display-link-issues-in-customer-portal/qaa-p/2422678#M142438