Found this while making an example file. Doesn't affect me, but it might cause problems elsewhere.
[attach=1]
tgd attached.
I've had this too, even when using an image mask to cut them out. I've adjusted the contrast and used different filters to get a sharper line on the mask but it doesn't seem to make any difference. I'm guessing the bounding box isn't actually a binding parameter for the population otherwise you'd have boxes of trees or grass in every render but there must be a decent way to get around it, maybe with a distance shader.
I have this happen all the time. Seems to make no sense. There must be a better way to place plants where they are wanted.