I have been working on resolving this issue for the past week, but the accepted solution does not work in my case. Therefore, I am sharing some of my findings based on my experience while troubleshooting this issue:

1. Custom Policies Requirement: Initially, I was using user flows to address this issue. However, I later discovered that single logout functionality only works with custom policies.

2. Cookie Sharing in Third-Party Contexts: By default, cookies are not intentionally shared in a third-party context. This means that requests within an iframe will function as expected. Additionally, the Secure attribute must be explicitly set for the cookie to ensure proper behavior.so, cookie sameSite attribute must be none.

Below are the cookie configuration images for both the .NET Core application and the legacy application that I have implemented.

.Net Core Configuration

.Net 4.8 Configuration