Okay, so for anyone who has this issue, this is the only workaround I have managed to find:
Before you enable edit layers on the landscape, you have to use the visibility brush to make part of the landscape invisible. After this, you can turn on edit layers, and the visibility brush should work fine.
If it still doesn’t work, I have found that changing the Navigation Geometry Gathering Mode to instant or lazy might do the trick.
Hope this can help someone 