Terrain Details are culled at a short distance even though the Detail Distance in the Nature Renderer component is very large.
If the detail object contains a LOD Group then the culling distance of that LOD Group is used instead of the Detail Distance in the Nature Renderer component.
Increase the culling distance in the LOD Group to ensure that your terrain details are culled at the correct distance. Note that you can never set the culling distance of the LOD Group further away than the Detail Distance in the Nature Renderer component. The Detail Distance is always the maximum allowed distance.
LOD Groups override the Detail Distance of the Nature Renderer component so that you can control the culling distance for each detail object individually. This allows you to render large terrain details from far away, while culling small objects at a short distance.
For more information on LOD Groups, see the LOD Group page in the Unity documentation.