No FAQ. No Bungie Employee post. But that is what is assumed. Social gametypes have very lax matchmaking perimeters due to people being able to both bring in guests (who have no trueskill level really) and being able to be on both teams in one party.
So people get split. In Big Team Battle in particular parties are noted for getting split the most. You know how BTB is 16 people? Sometimes it'll match up with just 14. In which case 7 on a team and one stranded ( I could be mistaken, but I think it tries to still give the lone person an extra party member... but perhaps that's in Reach I'm thinking of). That's why in a lot of BTB parties you'll get one person designated to 'back out' at a certain point, allowing the team to restart searching and find a new game of 8v8.
In the past, parties of, say, 5 in the 5v5 Social Slayer were getting split up. It does happen in Halo 3, and it isn't too rare an event. But after these reports started coming in, Shishka (the person who 'ran' matchmaking at the time) came out and said that was not intentional- it was an error on their part, and asked for people to give examples so he could find the solution.
To be honest, I'm not sure if that was ever fixed. But either way, it shows that getting split could happen either way- either the way you described, or by a glitch where teams would just randomly get split.
From my understanding anyways.