Alpha channels in .png do not work as intended, also not in Cycles node. Blender has, I understand, same issue.

One solution is to encode the transparency in your texture: let pure black (rgb=0,0,0) represent transparent and any other color opaque. Use a math.step node to filter. Signal will be white(=1) for transparent so for use in Posersurface root node you will need to invert it, by subtracting it from 1.