Only first file is webp

Below you can see the generated html code on my site. As you can see only the first file in the webp source tag is also a webp file, all others are still jpg? In my example both the file for 325w and 500w should be webp. Am i doing something wrong here or is this a bug?

I use a default imageapi_optimize pipeline for all images. Where i setup two prozessors (JpegTran and WebP).

<picture>
  <!--[if IE 9]><video style="display: none;"><![endif]-->
  <source srcset="/sites/default/files/styles/max_325x325/public/referenzen/20140718_095814.webp?itok=7Vqt-fJ3 325w, /sites/default/files/styles/max_650x650/public/referenzen/20140718_095814.jpg?itok=M-O8Htez 500w" type="image/webp" sizes="(min-width: 1290px) 325px, (min-width: 851px) 25vw, (min-width: 560px) 50vw, 100vw">
  <source srcset="/sites/default/files/styles/max_325x325/public/referenzen/20140718_095814.jpg?itok=7Vqt-fJ3 325w, /sites/default/files/styles/max_650x650/public/referenzen/20140718_095814.jpg?itok=M-O8Htez 500w" type="image/jpeg" sizes="(min-width: 1290px) 325px, (min-width: 851px) 25vw, (min-width: 560px) 50vw, 100vw">
  <!--[if IE 9]></video><![endif]-->
  <img srcset="/sites/default/files/styles/max_325x325/public/referenzen/20140718_095814.jpg?itok=7Vqt-fJ3 325w, /sites/default/files/styles/max_650x650/public/referenzen/20140718_095814.jpg?itok=M-O8Htez 500w" sizes="(min-width: 1290px) 325px, (min-width: 851px) 25vw, (min-width: 560px) 50vw, 100vw" src="/sites/default/files/styles/max_325x325/public/referenzen/20140718_095814.jpg?itok=7Vqt-fJ3" typeof="foaf:Image">
</picture>