Have you got SAML tracer available by any chance? (https://chrome.google.com/webstore/detail/saml-tracer/mpdajninpobndbfcldcmbpnnbhibjmch?hl=en - also available for Firefox)
That should give you insight in what the exact message is that you're getting back from Keycloak. Have a look specifically at the "InResponseTo=" field in the response and compare it with the "ID=" field in the original request from the F5 to Keycloak.
There may be some more useful information here; https://support.f5.com/csp/article/K05876945