So preview grids seems to be broken in 5.0.1. The cells get really skewed depending on the camera angle, and swim over the surfaces.
Also, things cull unpredictably. If you just open a 3rd person starter map, enable partition, and place a bunch of cubes scattered around the level, and PIE, some cubes will not cull until waaaaay farther away than the loading range (tested with cell size at 1024, loading range at 2048 and World Partition Editor cell size at 12800, as well as various other numbers). After testing several layouts of cubes, the ones that don’t work as expected seem pretty random. Much of the time, the problematic cubes will act as a “culling group,” that is, they will all cull and show at the same distance, which is not perceivably related to the loading range, or cell size.