As soon as the bar at the bottom turns blue and the mode has been loaded go to the social menu → fireteam.
You will see map and gamemode on the right side.
If it’s not the one you want, hit the X button (at least on x-box to leave the fireteam) to leave the fireteam.
Now you can start the process again
Won’t this still count as a player disconnect/loss?
I left 10 in a row this Way and didn’t receive a ban.
You can do a similar thing when the game first loads, you can pause and check the game mode before the spartan circle and you can leave there too without getting banned
This is matchmaking manipulation, and after the grace period ends accounts will receive bans. I’d recommend not doing this.