Slide background image mixes with background image of another slide

I'm using Captivate 2017 on a PC.

I'm getting a strange effect when playing a screen recording demo which must be a bug.

In the demo there is a series of 3 slides. Each slide has a background image - for example Background46, Background47, and  Background48. These are still images, not full motion swfs.

Each slide shows its own image correctly when looking at them individually and playing them on the timeline.

But when I publish as a video or play the series in the previewer, Background46 displays correctly,  Background47 doesn't display at all, and parts of Background46 appear behind Background48 - as if there were holes or transparent areas in 48 where 46 shows through.

I checked these image in Photoshop and there are no transparent areas in them.

What is going on and is there a way to fix it?