I encountered this issue while experimenting. In the attached file I have a polysurface (red) that would not boolean union or even split. I did an intersection (purple) between the two and found that intersection was a closed curve.
I cannot split the polysurface using this close purple curve.
It appears the problem is the two conic-shaped curved at the right, top and bottom, that fair into the circular surface.
If I split the polysurface, with the curve it just fails.
If I explode this polysurface, I can split each of the surfaces (that intersect) individually and rejoin.
If I explode and try to split all the surfaces in one operation, I get two bad surfaces (the conic shaped).
The oddity here is that the cones are outside of the split line. I cannot split them individually using the curve because there is no intersection BUT, they get modified if I try to spit everything together.
Problem Split.3dm (3.1 MB)
This is what the conical shaped piece looks like after the split as a group.
So saved his funky file for posterity.
Your file contains only one polysurface. What surface/polysurface did you try to BooleanUnion it with?
The purple curve is open, not closed. It appears to be closed because the ends overlap. Use CrvStart and CrvEnd to find the open ends. Also, from Properties > Details (in Rhino for Windows) for the curve:
Geometry:
Valid curve.
Open polycurve with 8 curve segments.
Don’t assume a curve is closed without checking Properties > Details and/or using CrvStart and CrvEnd
My misteak. I must have saved this at the wrong state. I kept playing with it until I got what I wanted to work.
Here is another split issue. Hopefully I have saved the right file this time. I have an open surface (red) and two closed surfaces (purple):
If you split the red surface using the purple surfaces, at the opposite end, red transforms like this:
How might this be avoided?
It took me a long time to figure out where the problem was. I was building part and it was totally screwed up. I was not seeing how it got screwed up because the bad area was never in view.
Problem Split.3dm (3.7 MB)
That’s not a problem with the split. It’s the result of low render mesh settings. 
