cr004d are you running 7.5 or 7.5.1?
For some reason, I recall that the JSON DPH in 7.5 did not handle the HTTP headers when recording from R/R pairs. It worked for live system recording. And, I believe the 7.5.1 upgrade resolved the issue. Sorry, but I no longer have an instance of 7.5 to verify against.
I recall creating a DPH to read JSON R/R pairs, construct a Basic HTTP Request, and parse the headers into DevTest meta data properties. I believe it was for 7.5, but it could have been for a 6.x or 7.1 version.
I would encourage anyone out there to correct me if I am wrong so that you are not misled.