I have a rather large terrain on top of which I have tiled several instances of the same procedural foliage spawners in a grid. The spawning rules are driven by a landscape layer. The issue I am having is that there are discontinuities at the edge of each volume in the X direction, as pictured in the image below. The green checkerboard pattern is the driving landscape layer where the trees should be spawning.
EDIT: I have done further testing and am now even more convinced this is a bug. I have narrowed the issue down to have something to do with the inclusion landscape layers.
I created a debug layer with stripes on it for testing. At 100% opacity the trees spawn on that layer as you would expect.
Now, if I drop the opacity on the landscape layer mask and re-import it, and resim the foliage, the foliage stops spawning halfway across.
I do not believe this is some sort of subtle interaction with other landscape layers, because the artifact ‘follows’ the foliage volume if I move the volume and resim.
So to sum this up: Procedural foliage spawners appear to break when spawning onto a landscape layer which is only partially opaque.