Can't display images on Stage Display — "Current Slide" layer can't be re-created

I've run into a problem in Presenter that I can't solve. In one of my services I added songs and images. Text shows up correctly on both the main output and the Stage Display, but images only appear on the main output - the Stage Display stays black for them.
While troubleshooting, I noticed that one of the templates contains a layer element called "Current Slide." As I understand it, this element is meant to display the actual slide content - images or video - on the Stage Display. That seems to be the mechanism that should make images appear there.
The problem: I can't get this element to work, and I can't re-create it from scratch. If I go to Add New Template → Stage Display, it's generated with a preset layout that already includes "Current Slide" and "Next Slide." But if I delete those layers and try to rebuild the template from zero, the "+" button in Template Layers doesn't offer any option to add a "Current Slide" layer back.
So the mechanism appears to exist, but there's no way to add this layer manually - and I couldn't find any combination of templates or settings that gets an image to actually show on the Stage Display.
Is this a bug, or am I missing something? Ideally, the "Current Slide" layer should be available in the "+" menu so it can be added to any Stage Display template.