we have noticed something strange. Every time we refresh an Archive page with images on it, in cpanel we have noticed that the last Modified date is updated.
We use an Archive with a content template which uses the following code
To confirm, If I understand this correctly, the images which have already been uploaded in the media uploads folders, show the updated "Last Modified" dates, simply when the website's homepage is viewed?
If this is correct, I would recommend testing the homepage with all non-Toolset plugins disabled and also with a default theme like Twenty Nineteen. This will help in narrowing down to any possible interference from a third-party plugin or the theme.
In case the issue still persists when only Toolset plugins and a default theme is active, I'll need a copy/snapshot of your website, for further troubleshooting.