Looks like a known issue, and is working as intended. The workaround is posted here Github Actions commenting @cursor not being detected
Use a Personal Access Token (PAT) instead of GITHUB_TOKEN in your code review agent’s workflow so the comment is posted as a real GitHub user rather than a bot/GitHub Actions identity. When comments are posted using GITHUB_TOKEN, they’re authored by GitHub Actions and go undetected by Cursor. Swapping that for a PAT makes the comment appear as a human user, which Cursor’s automation trigger picks up normally.