Mentions in inline tasks are ignored and not indexed in lucene as mentions. This is a change made in CONFDEV-8546 , which fixes a bug where the assignee of a task is being sent two emails (one for the mention, one for the task assignment). The fix for the bug CONFDEV-8546 made the first mention in the inline task an assignee, and ignored the other mentions in the task (i.e., not indexed). Therefore, searches for a user's mention will not return any pages where the mention exists as an assignee or as a secondary mention in the task.
GDPR removal of a user's mentions in the search index does not pick up those secondary mentions due to the above reason, and therefore the search results will continue to display the username in the excerpt.
Edit the page with the secondary mention in tasks, which will force a reindex of the page, and fix the issue.