Try to precompose with camera inside the precomp and keep the layer in 2d, or a better idea, render only the grass in alpha and then composite with the cloud.You said “Interestingly, one thing i did come across is that if it passes a certain point in z space, and simultaneously in x space, thats when the edge disappears. if its one the other side of those points, its fine”,so just move/adjust your anchor point to adjust the z and x to the position you said is clean, with the cloud positioned where you want it to be.
Philippe Lessard
http://www.altproductions.ca