It was not a routing issue and the cause has finally been sorted.
After validating everything in the document and the setup in Azure the issue was discovered to be Anti-Spoofing.
The documentation states that Anti-Spoofing should be disabled on the frontend cluster interfaces (eth0). It does not however mention anything about disabling Anti-Spoofing on the backend cluster interfaces (eth1).
After going through the document again this morning I set a log filter for a source of the backend-lb, 168.63.129.16.
After a couple of iterations while working with support we finally came to the conclusion that Anti-Spoofing needed to be disabled on cluster internal interfaces also.
Policy was pushed after disabling Anti-Spoofing and everything started working as expected.
The documentation needs to be updated to also include disabling Anti-Spoofing on eth1.