My LSmate was in amdapor keep hard and on last boss. I tried once to join by selecting that only and made sure to check the box for que in progress. It failed and was trying to start a whole new dungeon. This happened twice. On the last try, I made sure everything was correct (searching for members, they still needed only a tank, and my DF box was still correct).

I was still unable to join their in progress dungeon.