Unable to reproduce.
I didn't try extending your staircase, but from your setup I can see that you wouldn't be able to; the staircase on the water level is flooded because you don't have a complete Moses Effect ring set up. Water is flowing onto the stairs from the north and south, causing the stairs to be at 3/7 or more depth and preventing passage, and your pumps on the east and west are insufficient to prevent this. I was able to build a wall on z=-18, and I set the miners to digging a spiral ramp down to the second cavern layer at ~z=-23 or so, then built a floor down there.
I suspect that you're cutting off your ramp or stair access when you continue the fort, which is what would cause the miners to fail to come back to the surface (because they can't). Also, your understanding of the Moses Effect is... incomplete, let's say; you appear to believe that pulling water out on some of the sides is sufficient to dry the middle, but you need to pull water out on all sides. 2 of 4 isn't enough. Were you to make more pumps to make a complete ring around where you want to build instead of just 2 walls, the floor should dry and nothing should prevent you from going through that lake there. Unfortunately, your fort is difficult to read due to how many z-levels it's spread across and the bad cavern gen so I can't give more specifics, but an educated guess suggests that you're trying to dig through water and/or inadvertently cutting off access routes.