Why is React loading failover img src if WebP is supported in Firefox?

My understanding is that given the following code, only the *.webp image should be loaded.

<picture>
  <source srcset="macos-1920x1080.webp" type="image/webp">
  <img src="macos-1920x1080.png" alt="..." width="100%">
</picture>

But somehow, create-react-app's loads macos-1920x1080.png, and then macos-1920x1080.webp when I start app using npm start (everything works as expected when using npm run build and serving app using Express).