More reason to use Firefox.
Edit: as pointed out below, it is probably CDNs to blame, not Chrome. Sorry for a false alarm (still going to continue using Firefox, though).
Could it be that the web page offered different formats and Chrome chose WebP? Developers can do this now:
<picture>
<source srcset="some-image.webp" type="image/webp">
<source srcset="some-image.jpg" type="image/jpeg">
<img src="some-image.jpg" alt="...">
</picture>
If Chrome chose to display the WebP image, that's what it will offer to save even if the <img> tag points to the JPEG version.