Have never seen this before , but when using plants from various sources, I have noted that some are single objects, with pivots place on the ground at the base... while others are groups and each part can have pivots centred to each and only when you use affect pivot and choose save the move tool, does the pivot for said group, move to it's proper position.. Are those trees that move, single objects, with multi materials or are they groups.. Maybe this could be the problem. if it is, then I would suggest, ungrouping said objects, then attaching each part to say the trunk, which normally has it's pivot at the correct point.. and if once al parts are attached to create a single object, if the pivot is not at the base of the plant, you can move it to where it should be.. Then reset transforms.
I know that if you use splines as areas for plants, each FP object, uses the spline at the highest level, so to do several levels you would need several splines, and in Max, when you render locally, al would be good.. I don't know if it's likely but could rebus be seeing the splines as a single area...
As I say, I've never seen this before, unless as in a recent tutorial, you were attempting to scatter on a displaced surface. but I have a feeling it's a grouped/single object/ pivot point problem