When swapping edges or faces, add the embedded faces as extra argument. When connecting the outside tets (just before the end of the routine), it might happen that two tets with an embedded face in between is inside the outside tets (but not in the cavity as this is checked indeed). When connecting these two outside tets, it is not checked that there is an embedded face in between them, so during the next iteration of mesh improvements, this embedded face might disappear.