It’s the first time it happens to me, the level started to have part of its landscape / foliage greyed out when I load it. I learned that I need to select and click Pin. It works.
However, I need to do this every time I load the level.
I did not / do not have this with other levels and landscapes.
This is a folder with all kinds of runtime generated content inside your project folder. Among the data stored there is the WorldPartition visibility data. But it is likely not required, a long shot.