Hi Evgene and Louis!
Thanks for your help!
@Evgene, Although, in theory, Render all pages in one HTML file (jQuery Mobile multi-page template) should fix the issue. It does not. I have selected Render all pages in one HTML file (jQuery Mobile multi-page template) for months. That has been my selection all the time I have worked on this project. I only tried un-checking today. I agree it should fix it. But it doesn't.
Perhaps, the issue is with the background being larger than the container - as Louis mentioned. This is necessary for the project, as the larger background gives a nice moving effect on phones, and is the correct size for tablets.
I can also try jpg instead of png. Will check tomorrow