generate fallback images

I'm trying https://image.nuxtjs.org/nuxt-picture but my results aren't matching the documentation.

The documentation says:

When you use modern formats like webp in the component, a fallback image with jpeg format will be generated and used as a fallback image for old browsers.

Therefore, I to use it as the docs suggested:

            <nuxt-picture
              :src="imgUrl(webpimage)"
              sizes="300 (webp),300:600 (jpeg),600:900"
            />

(the function just places in a webp image)

An example of the resulting HTML is something like:

<picture>
<source srcset="/_image/ipx/remote/_/w_900_cover/http://localhost/hdwp-content/user/pages/01.home/wolves.webp" media="(min-width: 600px)">
<source srcset="/_image/ipx/remote/jpeg/w_600_cover/http://localhost/hdwp-content/user/pages/01.home/wolves.webp" type="jpeg" media="(min-width: 300px)">
<source srcset="/_image/ipx/remote/webp/w_300_cover/http://localhost/hdwp-content/user/pages/01.home/wolves.webp" type="webp" media="">
<img alt="wolves" class="__nim_o" style="position: absolute; left: 0px; top: 0px; margin: 0px; width: 100%; height: 100%; object-fit: cover; object-position: center center; transition: opacity 800ms ease 0ms; opacity: 1;" src="/_image/ipx/remote/jpeg/w_300_cover/http://localhost/hdwp-content/user/pages/01.home/wolves.webp">
</picture>

I tested the output in Safari 13 (Catalina) and nothing showed -- meaning that the JPG fallback wasn't generated. I tried PNG as well with no luck.

Am I implementing it wrong? Is the output HTML wrong?

Start by reproducing the documented nuxt-picture example from https://image.nuxtjs.org/nuxt-picture with the generated HTML shown in the issue, then compare behavior in Safari 13. Trace how the JPEG or PNG fallback is selected and generated; done means the component renders an image in browsers without the modern format support.