Why a preview image is missing part of the scene

A preview image renders what reaches the graph's Output. Anything on a Viewer is left out.

That is why a graph can look complete while you build it and come out missing its ground or its backdrop in a thumbnail or a share image: the editor viewport draws Viewer branches, and nothing else does.

Viewers are not a mistake to fix. A static object standing beside animated geometry is often on a Viewer on purpose, because joining the two would force the static half through the animated path on some export targets.

If you want one image of the whole composed scene, join the branches into the Output in a copy of the graph and render that.