This sounds like a caching issue. Please try temporarily deactivating your caching plugin then test if the images load as expected. If so, you’ve found the issue!
You’ll likely need to exclude caching for logged-in users. Not all caching plugins offer this option as standard, but you can use the following code snippet, if needed.
We couldn’t reproduce the issue on our end, as all images are loading correctly. As ChrisB mentioned, please try disabling caching plugins. If you’re using one, make sure that caching is disabled for logged-in users.
The issue persists, some images load properly, some don’t. A simple webpage refresh resolves the problem. Maybe it has got something with network time out in slider setting. This happens when caching is enabled or disabled.