If a Crucible review comment contains a text like "JA-1", it will be displayed as a "Linked Issue" under the review details, irrespective of whether this is an actual JIRA issue key or not.
Even if there is an entity link configured between a JIRA and crucible project and the propertyies "Disable for unmapped JIRA Projects" and "Force Project Mappings" has been set to "ON" in JIRA, the "Linked Issue" is still being displayed.
The actual text in the comment is also considered to be a JIRA link by default.
There appears to be no check in place to confirm the validity of the linked issue.
Screenshot attached.