The 'Seen' tags are showing up on thumbnails when I open a wallpaper in another tab but are gone when I refresh or load a different page (like my subscriptions). Looking into the LocalStorage using the dev console it seems that you're trying to store seen wallpapers as an array in 'wallhaven.seen_wallpapers' but every time I load a wallpaper it's being replaced by an array with only the new entry and even that is ignored on load.
I'm running Google Chrome Version 60.0.3080.6 (Official Build) dev (64-bit) on Windows 7.