Thank you for reaching out to us!
The Enable Seamless SSO is now only available for legacy tenants, for newer tenants this option is enabled by default, as mentioned in this documentation. After taking a look at your tenant, I can confirm that it is indeed enabled for it as well.
Another reason for SSO not working between applications is that the apps do not have the same Connection enabled. Please check and make sure you have at least one Connection enabled for both apps and test again if SSO works.
You can find more information on our Knowledge Article here.
Hope this helped!
Gerald