As you mention you can can choose an installation directory during installation and that, obviously, should be where it gets installed. If that’s not happening then that’s obviously a bug. That being said the installation package always gets cached on your C drive. At some point I will optimize this so that it doesn’t store the entire package but just a small subset needed to uninstall the game.
I don’t suppose you can post your installation log or email it to us? They are located in %temp%.