New answers tagged add-image-size
This is more of a general srcset and browser question, not a WordPress specific question. However, in general, you can't predict what browsers will do with the srcset information. For your specific example, it appears that you failed to clear the cache completely between your reloads. Note the 304 response indicating that your browser already has the image ...
After some intensive Googling I discovered a couple of plugins for just this purpose. This is the one I used: Regenerate Thumbnails
Doing this on your own is actually quite a lot of work. For the sole purpose of regenerating your custom image sizes I would recommend using this plugin: AJAX Thumbnail Rebuild It allows you to loop through your Image Library and regenerate the different image sizes that you selected.
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