I've been match made into a couple of missions that we're already in progress even though it's my first time attempting the mission. I would rather play the entire mission alone than to start it half way through.
Hmm... It's not supposed to work that way... There's a short window where you can still be match made into a mission but it should only be like a minute or so.
If you're coming in a significant amount of time after the mission has started, that's definitely a bug. (A a note, Stronghold matchmaking windows are quite different). 😊
Is that what you're seeing?