The recommended approach is to use an overlay instead of scene or sceneAfter, like this...
var geom = new THREE.SphereGeometry(10, 8, 8);
var material = new THREE.MeshBasicMaterial({ color: 0xff0000 });
var sphereMesh = new THREE.Mesh(geom, material);
sphereMesh.position.set(1, 2, 3);
viewer.overlays.addMesh(sphereMesh, 'custom-scene');
For more details, follow the example here: https://forge.autodesk.com/en/docs/viewer/v7/developers_guide/advanced_options/custom-geometry/
Thank you so much Michael, I was blind :-). Now it's working good.