Use root_url for issue boards user link

Closes #23556

CHANGELOG.md

@@ -7,7 +7,6 @@ Please view this file on the master branch, on stable branches it's out of date.
   - Add hover to trash icon in notes !7008 (blackst0ne)
   - Simpler arguments passed to named_route on toggle_award_url helper method
   - Fix: Backup restore doesn't clear cache
-
   - Use MergeRequestsClosingIssues cache data on Issue#closed_by_merge_requests method
   - Fix documents and comments on Build API `scope`
 
@@ -44,6 +43,7 @@ Please view this file on the master branch, on stable branches it's out of date.
   - Clarify documentation for Runners API (Gennady Trafimenkov)
   - The instrumentation for Banzai::Renderer has been restored
   - Change user & group landing page routing from /u/:username to /:username
+  - Fixed issue boards user link when in subdirectory
   - Added documentation for .gitattributes files
   - Move Pipeline Metrics to separate worker
   - AbstractReferenceFilter caches project_refs on RequestStore when active

app/views/projects/boards/components/_card.html.haml

@@ -26,7 +26,7 @@
         ":title" => "label.description",
         data: { container: 'body' } }
         {{ label.title }}
-      %a.has-tooltip{ ":href" => "'/' + issue.assignee.username",
+      %a.has-tooltip{ ":href" => "'#{root_path}' + issue.assignee.username",
         ":title" => "'Assigned to ' + issue.assignee.name",
         "v-if" => "issue.assignee",
         data: { container: 'body' } }