I made some tests and this is definitively bound to the upper distance of the frustrum beyond which objects are not displayed in 3D. The astonishing thing is that this distance is the same in onscreen (3D view) and offscreen (i.e. Q1 and Q2 photos) modes, but your objects are very close to the limit set in Sweet Home 3D. If you move away the observer a few hundred meters more away from the scene, they will disappear too.
I could increase this upper distance to make far objects viewable, but then there would be some risks that objects in the nearby of the camera wouldn't look correct
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Emmanuel Puybaret, Sweet Home 3D creator
