The problem is because the leading edge is wider in a forard position, at a given posture this means you're digging into your flesh more with the saddle as you pedal downwards. You can demonstrate this by slumping on the floor against a wall, putting your hand on your ass and doing a pedalling motion parallel to the ground. Move your hand down towards your thigh as you pedal you will feel it digging in.
If you've sensitive skin this can lead to the discomfort.
The solutions I can think of are either to move current saddle towards aft of bike a bit, or get a saddle with the leading edge further back. Or leave saddle as is, get a longer stem and train yourself to be more stretched forward so the leading edge doesn't dig in.