It's probably because of this RFC Show archive.org snapshot :
Clients SHOULD NOT include a Referer header field in a (non-secure) HTTP request if the referring page was transferred with a secure protocol.
It's not clear why this SHOULD
exists. One might think it's because of session IDs encoded in the URL (something that was in fashion in the 90s), but then again browsers do send referers when linking from HTTPS to HTTPS.
<META>
tag
Show archive.org snapshot
you can use in some modern browsers.