I may be able to shed some light on this, but I have only been using World Partition a short time, so I will not claim to be an expert.
Creating a fairly large (circa 60 sq. km) landscape, I found that some of the components were invisible. I initially assumed it was something wrong with my landscape, or with my viewport clipping settings, but what was actually happening is that the GPU VRAM streaming pool was being exhausted.
In my situation, enabling World Partition fixed the problem as long as I don’t try to force WP to load all the cells at once, but there is also a console command and/or INI file setting to increase or to completely unconstrain the streaming pool size. (There are enough pros and cons of this setting that, rather than summarize here, I will simply suggest searching on that topic and reading the relevant posts.)
TL;DR: It may not be your landscape that is the problem, but simply a rendering issue. Depending on how much VRAM you have, the size of your splatmap textures, the complexity of your materials, etc., the engine may not be rendering certain cells.
I hope this gives you something else to look into. I apologize for not being more specific, but since I myself am still learning World Partition I didn’t want to risk giving you wrong info.