I'm currently working on the same thing (body tracking + unity + orbbec Astra Pro). According to the tests, I get similar results. The skeleton disappears at a distance of 4 meters (perhaps 4.5).
The Astra Pro and Astra have a depth range between 0.6 - 8 meters (the optimum range between 0,8 - 6), so I suppose it is by Unity, or by the application of body tracking, which may have certain limitations.
I also did body tracking tests with Kinect v1 and the results were similar. The skeleton disappears at a distance of 4 or 4.5 meters.
I don´t know if this distance could be extended. In my case, my application does not require greater distances.