Text does not show on sermon slides when built using user uploaded media

I'm using the beta build on my iPad - 29.0.0.(26) - although this is not specifically a beta issue as it appears in the latest stable release - 28.0.4 (33)

I uploaded a slide with a yellow background using the Media Tool and then used it as the background for some sermon slides. On the desktop slides appear as expected with the yellow background and black text.

But on the iPad, the text doesn't show

As far as I can see this is only an issue with user-uploaded media.

Is this a known problem, is there something I can do to work around it?

Thanks, Graham