Section cuts not showing up in scenes in sketch up

The Style that is saved with a Scene ultimately controls how the model is rendered in that Scene.
However, the way an active model is rendered onscreen is contingent upon two factors…
…the active Style and any temporary unsaved changes one may have made to that Style.

When a Scene is created the only Style properties that are saved are those of the active Style.
Any unsaved changes one has made to the active Style are not saved with the Scene.

Consequently…
Creating a Scene is not WYSIWYG unless the active Style is updated first, thereby incorporating any unsaved changes you have made to the active Style.

Like TIG suggests, before creating a Scene; consult the Styles Browser.
If unsaved changes have been made to the active Style you’ll see update arrows superimposed upon the style thumbnail image.

When saving Layers, Section Planes and Styles in your Scenes it’s best to create Scenes using the Scene Manager (expanded) rather than View > Animation menu or Scene Tab > Context Menu.
Keep the Styles Browser and Layers Manager open so you’re aware and in control of what’s being saved with the Scene.