It turns out that YouTube's mobile app for Android now caches the videos you watch. If you watch a video again, YouTube no longer has to download the same chunked files: it uses the cache. After watching a video, I switched to the airplane mode and I could play the video offline, even if it's not supported by YouTube Music Key.
Even YouTube's desktop site started to cache videos again. Since switching to adaptive streams (DASH) in the HTML5 player, YouTube downloaded videos every time you watched them, wasting a lot of bandwidth.