I noticed this was the case in a vanilla AEM 6.5 instance and did some digging and blogging.
Basically the template that renders the content fragment is missing the cmp-image css class that makes images responsive.
If you would like to read how I came to this answer please read more below:
Why are images in Content Fragments not responsive? | 约瑟
Hope you find this useful,
Thanks for Sharing.
Could you please raise this issue for core components on GitHub Issues · adobe/aem-core-wcm-components · GitHub
Will do : )