Even with a deck stepped mast the mast post may leak depending on design and what is above. On my old Beneteau, there were wire connectors above mast compression post and also it's own bolts actually went through the deck. The area was covered with liner so it wasn't obvious immediately where the water intrusion is, but it did actually leak both into and outside the compression post and then down into the bilge. A proper cleaning and resealing of all bolts and fixtures solved the issue
So, it is still an area worth checking.