Hi,
The results of the power spectrum will be negative if the power flow through the observation area is in the negative direction along the axis. So in the case of an XY observation area power flowing along the negative z direction will be registered as a negative.
I would like to note that you are discussing the Py spectrum but in the design you attached the only observation area is a Z-Directed or XY observation area which means you are looking at Pz. Also that spectrum is all positive as expected from your design.
Scott