When using the "Route Via HTTP(S) Assertion" and selecting an unrelated certificate the backend service connection succeeds without error. The expected behavior is that this connection would fail.
It is a bug.
CA API Gateway - 9.3(or lower)
This is happening because Global Root CA imported into the Gateway and is marked as Trust Anchor.
The workaround is to unmark Global Root as a Trust Anchor or just remove the Global Root CA certificate from the Trusted Certificates.
CA will provide the fix in the next release 9.4.