This is an unfortunate part of how the the Atlassian apps framework works, running code in an iframe on a different domain to your Atlassian instance.
When Safari has “Prevent cross-site tracking” enabled, it prevents an iframe on another domain from using third-party content providers to display information.
This can a good thing when the cross-site tracking is used by advertising networks to track your behaviour across multiple site. However, in this case, it can prevent our app to sign in correctly or display data.
Use another browser
Ask your users to use any of the other browsers which Atlassian supports such as…
Update your Safari settings
If the your users uncheck the “Prevent cross-site tracking” setting, then the app should work correctly.
From the Safari menu, choose Settings… (or keyboard shortcut ⌘ + ,)
Then select Privacy and uncheck Website tracking: Prevent cross-site tracking