Dear Oliver,
Thank you for your reply. Vds is always positive as the DC link voltage does not change, therefore there is no need to mirror the voltage in the losses maps. However, depending on the duty cycle the current across the MOSFETs can be positive or negative (using PLECS convention). For example if the half bridge is working as a buck converter, the current in the top MOSFET goes from drain to source (positive) during ON time, but the current in the bottom MOSFET goes from source to drain (negative) during OFF time. I mirrored my maps to account for negative currents but I keep getting zero switching losses whenever the current is negative.