Landscape Splines in Partially Loaded World Partition Levels Can Cause Nanite Landscape Invalidation

Certain placements of landscape splines in world partition levels can cause Nanite landscape proxies to invalidate, and switch back to their non-nanite version. This happens when specific regions of the world partition level are loaded, while others are not.

This issue appears to be related to the following fix for landscape determinism:
https://github.com/EpicGames/UnrealEngine/commit/6bb85f022768e4ca80afe4b10368603f1358558c
This fixed a very similar issue that occurred even when all regions were loaded in a world partition level. This issue caused the landscape proxy nanite data to inconsistently invalidate after the level was re-opened several times. The issue affected the same landscape proxies that are affected by the current issue when using the same landscape spline. The main difference is that now this issue only occurs when a world partition level is partially loaded. Loading only the region that contains the affected landscape proxy is usually sufficient to reproduce the issue.

Note that this issue is difficult to reproduce without the provided reproduction project. The issue only seems to trigger with certain landscape spline placements, and only on certain landscape proxies that are affected by said spline. Therefore, these reproduction steps assume you are working from the provided reproduction project.
Note also that I will be using an alphanumeric grid coordinate system (e.g. region D3), with numbers vertical, and alphabet horizontal, as a simple way to refer to specific world partition regions. I will consider 1 region cell to be the size of a single landscape proxy in the default open world level (clicking on a landscape proxy in editor will show how big this is on the world partition minimap).

How the reproduction project was setup:

• Create a new default Open World level
• Open the world partition editor window and click Build -> HLODs -> Delete HLODs (this was just to make some things easier to see, and to eliminate variables. Should be irrelevant for actual reproduction)
• Switch to Landscape Mode
• Under Manage -> Splines, add a new Landscape Edit Layer Splines layer
• Ctrl + Click in the viewport to add connected spline points. Add many of these around the entire map.
• Switch back to Selection Mode.
• Select the LandscapeSplineActor and move it up in Z slightly so that the landscape is noticeably deformed by the spline everywhere.
• Select the Landscape and tick Enable Nanite, followed by Build Data in the details panel.
• Save everything (ensuring all world partition regions are loaded when this is done)

Replicating from the reproduction project:

• Open the BestRepro level from the supplied reproduction project
• To ensure the level is prepared properly:
• Ensure that all world partition regions are loaded
• Select the Landscape and click the Rebuild Data button under the Nanite category
• Save Everything
• Enable the Nanite Overview view mode
• Observe that all Landscape Proxies show up in the Nanite Overview visualization. Showing they contain valid Nanite data.
• Unload all world partition regions (Box select all regions in the World Partition editor window, right click -> Unload Selected Region(s))
• Box select the inside (careful not to select outside this region as we only want to load a single landscape proxy) of region D3 in the World Partition editor window, right click -> Load Region from Selection.
• Observe that the Nanite visualization flickers from, first showing the loaded proxy with valid nanite data, to black, indicating no valid nanite data. There will also be a warning in the top right of the viewport saying: “1 Landscape actor is out of date and needs to be rebuilt”.

The same issue can also be replicated using region C3. However, no other regions seem to exhibit the issue. Specifically, if either C3 or D3 are loaded, but not the other, then the issue will occur (regardless of the loaded state of other regions). Both regions must be present in order for Nanite data to stay valid.