The package that contains the PCGGrapch is dirtied on load (UPackage::SetDirtyFlag) which triggers a resave operation. I traced it back to the following callstack: UnrealEditor-CoreUObject.dll!UPac ...
In my example picture I have "Grass" set as the target attribute area but the debug cubes are appearing on the dirt and snow as well. This only happens when the landscape subsections are set to 2x ...