Landscape cells not active in Play Mode

So the first image is how things stand in edit mode and the second image how things are in Play mode. Several cells are just not activated. All cells loaded via the World Partition window. Cant for the life of me see why these cells are inactive, I have nothing therein except a textured Landscape and a couple of Third Player Characters. Other than that basic lighting. Its a largish landscape though if that has a bearing 12751 verts sq. Any thoughts?

Further to this I notice that only a small number of cells are showing during play mode, 7 out of the 169 in the level set. And as you can see this is not just immeditely adjacent cells that are loading but rather an apparently random subset!
Any help appreciated guys!

So started afresh - new project - only imported the heightmap. No other edits.

Many cells not loading in Play Mode, the same seven only loading, they seem fine in the Editor viewport. Help!

So also I notice that if I drop the size of heightmap to 8161px sq - it seems culling distance is kicking in and limiting to just a tile in each direction from the character. Do I need to set a value for this somewhere? Is it related to the above? Again only 4 LandscapeStreamingProxies are active during Run-time, out of 64.
just for the record I am also getting “Landscape physical material needs to be rebuilt”? - yet all I have done is import a heightmap and hit play!

In answer to my own questions then.
Loading Range was the issue - upped the value by a factor of 10 - problem gone!
Loading Range

My concern, and reason for a related question on the boards, is that the world partition cells in UE5 also will load/unload the actors you place on them. Once you populate your world with actors I believe it will load all actors in all the cells within the loading range. Unless you get fancy with data layers, I think you will want to be careful with just how far that distance is. I was hoping to find a way to always display landscape cells containing distant mountains to help offset this problem, but have not figured out if its possible.