It happens to every forest system, some using the distribution map and area splines and some using explicit points. It would appear that they are being reset to no rotation and scaling. I can't say if we had the problem before as we had additional problems on old versions that would have superseded this issue. Otherwise, we're rendering tens of thousands of trees pretty effortlessly!
As you can see in the two bad frames, the trees are all very aligned and uniformly scaled, whereas in the good frame they are nicely randomized.