First, note that 3D models are handled as POI objects in VisioWeb (like images and texts).
This is the default management for POI opacity. When you zoom out, the POI objects will become semi-transparent as a function of the distance to the camera until they disappear completely. This is especially useful for large maps with a lot of small objects because it helps saving some performance when the user zooms out.
But for 3D models that represent potentially huge buildings, this default behavior might be inappropriate.
Using VisioWeb >= 1.8.1, you will be able to set the opacity of such models with a constant:
var model = mapviewer.getPoi('my-model-id'); // getPoi returns an array of POIs model.opacity = 1;