The correct overburden stress will be calculated if a reservoir is modeled using a no-strength material with a unit weight of water. If a pressure line is used instead, the weight of the water will not result in an increase to the overburden stress. Since the Shansep model uses the computed overburden stress to determine the strength of the soil, using a pressure line will result in the strength of the soil being underestimated. This will also result in a lower factor of safety.
In SLOPE/W, individual line loads are not considered as adding to the overburden stress for a soil assigned a Shansep (S=fn(overburden) strength model. When a pressure line is used to define a pressure over a given area, the pressure is converted into a series of line loads that act on the slices that exist beneath the pressure boundary. If the slices contain a soil that has been assigned a Shansep strength model, then the influence of these line loads on determining the strength of the soil will not be considered.
If you model the reservoir as a no-strength material, then the program will automatically include the weight of the water and use this weight together with the defined tau/sigma ratio to determine what the strength of the underlying soil should be.
This is not a bug in the software. Modeling a water layer with a pressure line was a useful option in version 5 because there were restrictions around how the no-strength soil model could be used and how it had to be incorporated into the geometry. In version 6 these restrictions have been removed so it is unnecessary to ever model water with a pressure line. It is always more appropriate to use the no-strength soil model to represent a water layer.