I finally managed to capture the broken lobbies on video.
One Fireteam member doesn’t get the Callsign (A, B, C, D), while one team member becomes either invisible/Bot, or gets the Foxtrott Callsign.
Players then experience the Blackscreen, once the Mission Briefing starts.
Happens up to 50% when entering random Crossplay lobbies.
On non-Crossplay Lobbies, this doesn’t happen as much, though it still happens there too.