We followed the same steps as per the document but we are unable to authenticate to specific realm in AM.
Could you please let me know if it is a BUG? When we are authenticating to root realm its working.
Can you elaborate more on what sort of issue you are facing? Are you seeing any specific errors? Are there log files with any messages you could share? I see you are referencing the /alpha realm - are you using Identity Cloud? Or is this an on-premise (or otherwise hosted) Identity Platform deployment?
I am not aware of any issues with using IG as a PEP, so I suspect that there is something configuration related at play and we can troubleshoot it here.