I have seen this issue on other versions on macOS as well; I don't think this is specific to this version. You will need to trust the certificate when on the site click the padlock. Show the certificate in the details you should see the CA Issuers, click on the URI. (This will ensure the correct cert required is downloaded) You can then add this certificate into keychain.
Topic:
App & System Services
SubTopic:
Core OS
Tags: