The launcher and the game have their own independent connections to Blizzard services I would imagine.
You can exit the launcher entirely while the game is running for example and the game continues to run independently just fine and you can still use chat (assuming you aren’t experiencing the issue discussed in this thread).
As I already mentioned, when you are disconnected from Blizzard services in-game, the launcher chat still works just fine. You can chat in group chats in the launcher while unable to do so in-game. Your launcher friends list works while your in-game one does not. You can whisper people via the launcher but not in-game.
I’m pretty confident they are already aware of the issue and that the issue is likely affecting thousands of people since it seems to be a bigger problem on larger servers (just basing that anecdotally on what other people are saying).
My theory is they made some changes for TWW and didn’t properly test them with larger pop servers. Similar to how the AH database change issues came about. There is probably some technical limit/hurdle on larger servers with how they interact with Blizzard services.