Fallback SRC not appearing

Hi

I've discovered an bug where the fallback src attribute is not output in the <img> tag if you choose any breakpoint group other than the default 'Responsive Image'. For example, if you're using the Bartik theme and you choose the 'Bartik' breakpoint group in the Responsive Image Styles configuration page for your inline images, then when you place an image inline in CKEditor, the code that is output on the page uses the <picture> tag with the various <source> tags and a fall back <img> tag that only has the srcset attribute but not the src attribute.

Any ideas?

Thanks

F