Yeah three-bvh-csg using a clipping technique that results in “T-Vertices” - or a 1-to-many mapping between edges - so this is a limitation of three-bvh-csg, right now. From your uploaded it looks like this is what’s happening but the form otherwise looks solid.
In the future I would like to address this but I can’t say exactly when I’ll have the bandwidth. Some of the other CSG libraries out there (Manifold, three-csg-ts, etc) may produce better results for 3d printing at a slower speed. For now if you need real time results you could use three-bvh-csg for intermediate results and another library for the final trim.