The issue was addressed with improved routing of Safari-originated requests. This issue is fixed in macOS Sequoia 15.2, iOS 18.2 and iPadOS 18.2, Safari 18.2, iPadOS 17.7.3. On a device with Private Relay enabled, adding a website to the Safari Reading List may reveal the originating IP address to the website.
https://support.apple.com/en-us/121846
https://support.apple.com/en-us/121839