Given the enhancements made available to Safari as explained at Full Third-Party Cookie Blocking, cross-site tracking is disabled by default in recent versions of Safari.

This feature is now affecting third party apps that rely on Third-Party Cookie (e.g. using the current user's session to load content such as knowledge base articles within an iframe embedded in a vendor's web site).

As part of this enhancements, it's expected to have cross-site tracking disabled by default by other browsers too.

There's already use cases that rely on Atlassian's cookies which are already being affected in Safari and will be affected soon on other web browsers.

This blog post from webkit has some interesting suggestions on how to circumvent this issue: https://webkit.org/blog/10218/full-third-party-cookie-blocking-and-more/