I am troubleshooting this issue as well. Ishani DePillo, did you get any feedback from engineering?
To be honest, I have seen a lot of unpredictability in the original referrer field over time -- from it not populating at all, to sometimes picking up the actual referring page from the third party site to often picking up the initial landing page on the Marketo-tagged site. Sometimes I have tested identical scenarios and gotten different results each time.
It makes it difficult to rely upon.