Previously placed foliage suddenly under the terrain

I have been working on a large map with lots of foliage. After working on one end of the map for an afternoon, I noticed my foliage on the opposite side had sunk under the terrain.

In this image (trees hidden) you can see that it happens past a certain line:

I’ve been working with unreal for about a year now and the only time i’ve encountered this was because the terrain was being placed on another terrain, though this is not the case.
I tried reapply the z-axis variation in the foliage settings by regardless of what values I selected they would not affect the terrain.

Any assistance is greatly appreciated.