If you have a website with the HTTP protocol (also called non-secure website) you may receive inaccurate data about referrers.
This happens because of the referrer policy. The policy prevents sending the referrer data from a secure location (a website with the HTTPS protocol) to a less secure location (a website with the HTTP protocol). In that case, your web server doesn’t receive the referrer data and can’t pass that information to Piwik PRO.
If you are the owner of the website with the HTTPS protocol, you can change the policy and allow the website to send data to non-secure websites. Although, we don’t recommend that. It’s better to change all your non-secure websites to secure ones. Then you won’t have the issue with missing referrers.