See https://preload-resp-images.glitch.me/sample.html.
 

See https://preload-resp-images.glitch.me/sample.html. Essentially this describes that image preloads are considered "used", when referenced by <img>s appearing in a <picture>, even if the image resource in the <img srcset> is never actually displayed, and per spec, should never even be fetched. This said, I think the resource should not be considered a "matched preload" (thus used), and should trigger the "preload was not used within a few seconds of load event" warning.