Background image keeps going missing in presentations

I thought this was just on one presentation, but it seems to be on all of them. I just update to version 12.1 of Keynote thinking that would help, but it hasn't. I have used the standard template for Leather Book and Typeset and they open fine at first. It does seem to download about 6.2 MB before opening even though the presentations are set to keep on this device. After a few minutes of editing, the background image disappears from the slides and there's a tiny icon "question mark" where you select the background image. The original image was there before this happens. It seems like the images used in the template are stored online, so I saved a custom version of the theme. However, the same thing happened with that version. I haven't tried using a local image as the background. I opened the Keynote file as a Zip folder and couldn't find the background images stored in there. Any ideas?

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