Google this month introduced a new limitation on the vignetes. Users will only see this ad format once per hour, and per site.
Before, vignettes were only shown to users between page loads within the same site, but Google will start showing vignettes to users who are between page loads when they navigate to a different site.
Vignette ads is an ad format from Google Adsense. Vignette ads are mobile full-screen ads that appear between page loads and can be skipped by users. Google says vignette ads are displayed when the user leaves a page, rather than when they arrive on one, so the user doesn’t have to wait for them to load.