Wow, you found the reason. Thank you. It wasn't a bug, just a bad bit of design.
The circle and triangle were never intended to be concentric. The purpose of the circle was to create a segment on the side of the triangle - I messed up with orientation of the triangle which I regretted when I came to mirror the lobe. My intent was to project the segment onto the 45º plane defined on the triangle edge, and then move it around until I got the shape I wanted. But a circular segment turned out to be the wrong shape, so I changed it to be a spline, which I manipulated until I got the shape I wanted. I didn't notice that I missed a tiny section when I extruded the lobe, and that, as you have spotted, was the cause.
