Probably by opening it once, no idea how the app caches images. But it was more of a wild guess for a potential reason why it’s not working rather than a suggestion about what to do. Does it happen with every image or just that one? If it happens with every image then i guess it’s a problem with the app (or your internet connection), if it’s just this one then the image is probably not available anymore and it’s just showing the preview because it cached it before. In that case there’s probably nothing you can do.

I’d assume that the preview image was cached, but the high res version wasn’t. And now the image can’t be loaded but the preview still appears because it’s cached or something