When hopping into a stream for the first time you get an ad. There is then some cooldown before you get another ad even if you switch between streams.
On mobile, if you click a link in chat (for example, a clip from something that happened earlier in the stream), it redirects you to a different video player to play the clip.
The only way to return to the stream is to close this new video player, and go find the stream again. This then plays another ad because the clip took you away from the stream.
This is a minor inconvenience in the scope of all the issues that ads are bringing to Twitch, but this simply should not happen. Getting dragged away from the stream in the first place is strange, but getting punished for watching a clip of the stream is an awful user experience.