Well it doesn’t make sense to PRIVMSG a room you have not JOINed and JOIN specifies lowercase. So it sounds logical to me.
The thing with a whisper is that it’s interpreted slightly as a server command. So it’s parsed differently which is why it works without the colon. (I imagine)
Really if you have having issues. You may want to consider using an existing IRC library or twitch specific library. Rather than rolling your own