Me and my flight mates are also having a similar issue. I'm hosting the mission and always the first to lock into position on CATI or CATII. Any of my other flight mates that go to CATI or CATII, they bounce back as soon as they lock into position. I'm going to have my flight mates lock in first and I will lock in last to see what happens.
Update: Did some further testing today. This time around, I had a flight mate connect to CATI first...all good. I rolled up (I'm hosting the mission) to CATII. As soon as my "launch bar" touched the deck, my flight mate rolled back into the blast door. Even when we switch positions, he takes CATII and I take CATI, still happens. This is happening on two separate missions.