I'm not sure if this mod is the one responsible but i've been having this infuriating bug where the planner uses refined concrete when i try to build over water rather than landfill. Is there a chance that this mod could be responsible or at least provide one way to fix it?

Hey Geco - I was looking all over this and I finally found the cause. For me, I had Dredgeworks installed which replaces landfill with refined concrete. This mod also adds the ability for refined concrete to act as landfill and fill in water, but it uses 3 pieces of concrete to do that. You can change this setting by exiting to the main menu -> settings -> mod settings -> uncheck the refined concrete and landfill option.