As everyone else is saying, the scrolling feature itself seems to be randomized in the main menu.
Any input seems to be given just a random value (up, down, left, right, and left click).
Mouse input, WASD, and native arrow keys all seem to just GIVE you a random input while SHOWING you the desired input.
Kinda weird but it effectively makes selecting anything impossible, as it isn't "technically" selected as the menu shows you via highlight.
I guess the optomized path to getting the mode you want, every time, is to:
1: start game
2: immediately pick your desired mode
3: click start (so that when you go back to the main menu home screen you can just click Lobby and start from there
