Image blank time to time / 304 not modified

It’s something I ‘investigate’ since weeks and tried on different browsers / machines / ISP. And it’s back and back mostly in Chrome I would say.
There is nothing in my setting / pipeline that could lead to an issue with cache? Images are back in 2 cases :

  • when I clean the cash and reload
  • when I open the dev tool in browser : images are back without reload
    It’s annoying and I guess it’s related to hugo because images in /static has no issues. If in other location / external storage > problem.