Hello! Thank you all for reporting this issue. The bug shared here has been identified and fixed for 5.7. The regression impacts landscapes with more than 64 landscape components per landscape proxy. In most World Partition setups, the proxy component count is typically low (< 64 components per proxy) so nanite building appeared to be working as expected. In Non-World Partition setups there is only 1 Landscape Proxy and this bug resulted in only the first 64 components being built by nanite.
The fix will be in 5.7 but if you are building the engine from source you can implement the changes found here: https://github.com/EpicGames/UnrealEngine/commit/c2f796cb16783c89dcad1214d01d21df7a99564e
In the future, you can directly report a bug following these instructions: https://www.unrealengine.com/en-US/support/report-a-bug. Our team will receive direct bug reports faster than forum posts.
Thanks again,
Jared