in the zone planners right half of the map there's a rectangle made of a lot of water and some "sandy" edge/corner

Here you see a sharp edge between stone and water (except 1 row of stone over the water at the left side)
These pictures don't really fit (i also tested replacing the zone planner, it didn't help)

Probably fixed in 8.0.x

Replacing the zone planner won't help as it's a slightly hacky global cache of the map area.