This usually happens on the ads that promise 30 minutes of ad free music. After the ad stops, nothing plays. I found that I have to go to a different playlist, play that, and then go back to my intended playlist. After I go back to the playlist, the guy saying "what this video to enjoy 30 minutes of ad-free music" or "enjoy 30 minutes of ad-free music after this ad" (or someting along those lines) plays again. Instead of the ad playing, the music plays. Fortunately, it's a way to play the music, but it's a bit troublesome (compared to not having to go back and forth between playlists)

I also notice after the ad, the album artwork doesn't show up unless the phone's display is on lock/off. The only thing that shows is that banner that says "The next 30 minutes are ads free."