The highest object in an experience establishes the highest point from which the camera can view the experience. This is a boundary and it seems to hold true for the lowest object in the same way. When the viewer falls below the lowest object nothing is visible.
I suggest that an appropriate workaround be explained to all creators. The solution I have used is to create a sphere that has zero diameter and place this 10 meters above the highest point in the experience. The small diameter is to prevent it from casting a shadow. I named this object "Upper Limit Fake Object."
In the future, the Sansar team might develop a preferred solution that creators have not tried and provide leadership in this area.