"Trusted Server Certificates" of "Route Via Http (s)" does not work properly.

Document ID : KB000104011
Last Modified Date : 21/09/2018
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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.

CA API Gateway - 9.3(or lower)
It is a bug.
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.