APM Portal Access Rewriting

Hi all,

A customer of us is using the F5 with APM and a Full Weptop Config.
They're using a Full Webtop Portal with Portal Access Ressources and Rewriting.
One of the Portal Access is connecting to a Document Management Application (https://backend.com) where the Source IP is allowed for access from the F5.

Now the Backend Provider of the Document Management App has upgraded the Application and integrated a Keycloak IAM, since then the included OpenID Connect SSO Requests are not properly rewriten by the F5.
The Client connects then directly to the SSO URL for OIDC and not through the Portal Access Rewriting.

We created a HAR File on the client and see the following:

*******
Requests to rewriting:
https://portal.f5.com/f5-w-abc123/page1.html
https://portal.f5.com/f5-w-abc123/somejavascript.js
https://portal.f5.com/f5-w-abc123/somepic.jpg
.....

Then the following:
https://api.essentials.backend.com/api/userrequest

And:
https://id.backend.com/realms/essentials/protocol/openid-connect/auth?client_id......
*******

The obfuscated path /f5-w-abc123 results in the URL https://backend.com

So how can we have the other URL also rewritten so that https://api.essentials.backend.com/... is rewritten to https://portal.f5.com/f5-w-xyz123 ?

Thank you for your help