Also, this is through Grasshopper Python so that might be causing the error as well. See below for some screen shots. Here is the code:
if len(boundaries) != len(planes):
raise ValueError("ERROR: Boundary and Plane lists must have the same number of items!")
polylines = 
for i, b in enumerate(boundaries):
p = planes[i]
xform = Rhino.Geometry.Transform.ChangeBasis(p, Rhino.Geometry.Plane.WorldXY)
bb = b.GetBoundingBox(True)
You can see below that GetBoundingBox(True) is working just fine.
You can see in the next two images that both GetBoundingBox(Plane) and GetBoundingBox(Transform) are close but 1 box is located in the wrong location and rotation.
<img src=“https://aws1.discourse-cdn.com/mcneel/uploads/default/original/3X/5/8/588640284d8fb2c1527a34b3bec0fe42db12014b.jpg” width=“690” heigh t=“431”>
Finally, if I use the orient command from grasshopper to position the bounding boxes from the WorldXY plane to the new plane it is fixed But when I try to transform the boxes myself, they are incorrect(last image). It is close but not quite right.
Does this help at all? The last image is close… but it seems to distort the bounding box as it transforms it.