So I just ran a test on my end using minimal content to guarantee it has to do with the correlation between landscape grass output and the number of components in the landscape. My results do not exhibit the behavior you are reporting, which leads me to believe it could be something else. The difference between our projects are the meshes you are using and perhaps slightly different densities.
1 Component
100 Components
As you can see my landscape sizes and overall resolutions are identical to your test cases. Perhaps I am overlooking a step or you have left out a detail or project setting? My mesh is much simpler than yours and comes from the engine content with the world grid material applied.
Let me know if you have further details or I am perhaps missing something.
Thanks,