Uptime Monitoring is reporting the Site Health as Unknown
If your site is using Shield, then Uptime Monitoring won't be able to access the site. You can check that using cURL as follows -
curl -sLIXGET https://[site_url]/system/acquia-connector-status HTTP/2 401 www-authenticate: Basic realm=""
The 401 response and www-authenticate denotes that you need to provide authorization to Uptime Monitoring to access the site.
As you can read here https://docs.acquia.com/cloud-platform/manage/uptime/ - Uptime monitoring requests originate from the IP address
So you can allowlist this IP in the Shield settings and then the site will be accessed by the Monitoring System.