Besides the adobe defualt "referrer", we've set up an dimension to record any page refer to the landing page at hit level, internal domain is also included. Try to help us to see any "previous page" before the current one.
But in fact, it's nealy impossible the traffic from www.youtube.com directly, which represents the homepage of it.
Do you have any idea why this happens and any setting maybe wrong?
It's hard to say what's wrong with your implementation without knowing the details of how you're setting prop33. From your example values, I'd say that it's being set by a script that either returns the referrer's domain name, when the referrer just contains the domain name, otherwise it returns the full referral URL. But that's just my guess.