Hi, I’m struggling with the same issue, wondering if perhaps you found the solution? I assume that it is buggy on a rectangular landscape with unequal sides, but perhaps there is a way to make it work on such a world?
I had the problem, I guess atleast, after changing the material for my landscape. I had no water and the splines didnt effect the landscape. After updating from Ue5.7.2 to 5.8 and dragging a new Water Body River into the landscape it had water. A few materials were gone for my River but most importantly, there is a checkmark for “visable” if anyone ever has that problem. For affecting the landscape I had to go to the Landscape_WaterBrushManager and had to aktivate a few settings like “affects heightmap” or “affects visability layer”. I hope that it might help someone safe a bit of time.