Boolean difference - what are these artifacts?

I used BooleanDifference with three (3) columns to see the floor of a building and at the end, there are these 4 artifacts that are still connected to the polysurface ellipse.

When I try to trim them, they eventually delete the entire ellipse.

Can someone tell me what these are and how I can get rid of them?

Thank you.

What are these artifacts.3dm (951.4 KB)

Hi Edward - where the holes are tangent to the outer edge, there is zero thickness, which will cause problems.


@pascal Thank you so much.

I moved the outside columns in 5 feet and all is well.