I have had no experience removing the mast step, but I have experienced a ' drip' in this area . It turned out to be leakage , not through the mast step, but rather, water was passing through the aperture just a few inches aft of the mast step where the wiring for the mast systems passes though the deck. In my case , the water followed the wiring down and forward a few inches and dripped out of the cover plate which covers the aft rear of the mast inside the cabin.
Could this be your source too ?
Clare Jordan. Aragorn