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.