It appears to be linked to the new feature in Settings -> 'Privacy & Security' -> 'Protect me from malicious websites'
When that is unchecked, no longer does the warning show up when going to https://portal.office.com/adminportal/home. Notice this phishing warning happens after the credentials have been entered in. When entering the URL on the site https://www.phishtank.com/index.php, it is a rather recent entry into the phishing database. I am treating this as a false positive (YMMV).
Is there a way to whitelist specific URLs to not get captured by the phishing detection filter?