Ansys Employee

Boolean is only well-defined if at all points in the volume it is clear as to whether you are inside or outside of a solid.

Imagine two separate cubes sharing a vertex -- at the vertex is not clear where the volume is, part of one cube or part of another.

Same for two cubes sharing an edge.

Same for an elliptical body of revolution and a plane -- your case.

Either make them intersect a little bit, or make them separated -- you are making the CAD job too difficult by being point-touching like that.