Fix for this has already been suggested, so a quick tldr/bump:
1.) Make Shift+Place Blueprint add landfill underneath everything in water.
2.) Enable Placing Ghosts on top of Landfill Ghosts.
3.) Make a difference between Tile and Terrain, so you can blueprint path AND landfill.

When using BP there are different scenarios when placing on Land: 1.) Only Building: Works as expected. (Placed with a single use) 2.) Only Path: Works as expected. (Placed with a single use) 3.) Building + Path: Works as expected. (Both placed with a single use) So far so good, but when you try to ...