How to regenerate "grass map" from BP?

I have a terrain material with multiple layers using “grass” static instanced foliage and everything is working well.

In order to not show grass through the floor I paint a different terrain layer there using the “Editor apply spline” function in Blueprints. That all works, except that, I have to manually tap around the map in the paint tool, otherwise the grass is not updated (but the landscape texture is).

Below you can see grass through my floor, but the terrain layer has been painted with a"floor" layer that has 0 grass.

Now, after a single tap with the paint tool it works correctly.

Does anyone have a way to regenerate the “grass map” from Blueprints? Thanks!