On mobile, it goes to the last channel you were in now.
The rationale here is that you may be chatting with someone while browsing the forum. This allows you to go back and forth between chat and forum and stay in that conversation more easily.
If you tap the chat icon again, you go to the channel index, so there’s still an easy way to get there quickly.
The chat default channel id setting may be somewhat obsolete at this point (or maybe I’m forgetting something). @j.jaffeux / @chapoi do you think we can/should get rid of it?