When I sculpt my landscape, it changes visually, but game keeps invisible layer where the old landscape was

Hello,

As the title says, I want to sculpt my landscape. And when I create a dip in the landscape, let’s say a hole, I can visually see the hole but when I step over it, it’s like the old landscape is still there, invisible. An invisible layer that I can still walk on, and when I add Nav Mesh, it goes on top of this invisible layer, so enemies can walk over the hole as well.

Foliage also connects with this invisible layer. It’s very annoying, it wasn’t like this before, might it be a setting? See screenshot examples for better understanding.

In the example below, I used SMOOTH to lower the hill, but I can still walk over where the hill was before - see shadows. And it’s not that the foliage didn’t follow, I added this foliage AFTER I re-sculpted. Selecting it and pressing END doesn’t do anything because as far as it’s concerned, it’s already on the ground.