I also have a problem with lazyloading picture elements. It works as planned in Chrome, but in Safari (mobile version) and Firefox (on Android) the images won’t load.
By the way, I’m also using WebP images in combination with JPGs / PNGs, if that matters here…
I’m thinking your issue is a combination of the limited support for the picture element, and differences in browser support for WebP. As soon as I get support for the picture element working, I’ll let you know so you can see if that fixes it.