This was just happened in 5 minuses ago when I'm composing this post. I was disconnected in a middle of the fight of Titan Extreme. When I reconnected back to the game, my party members were completely changed.
Detail:
I was disconnected when I'm fighting Titan Extreme with one of my FC-mate in the middle of the fight with an error code 90002. So, as usual, I immediately reconnected back to the game ASAP but, unfortunately, I got stuck at the loading screen. (because the loading time is quite long. I saw a "Light Party" and "Full Party" message on the screen but the loading screen still didn't disappear). After a while, about 1-2 minuses, the loading screen disappear, it's still in the middle of the fight of Titan Extreme with a little progression. When I looked at the party member UI, I found that the party members are completely different. They were CHANGED/REPLACED COMPLETELY and my FC-mate did NOT in the party as well. (the is the weird part!). I can control my character and dodge the AoE. The fight ended very soon coz all my (NEW)teammates were kill in a couple seconds. The fight was reset and I got stuck at a black screen AGAIN. After ~30s, I saw a msg "Motion to abandon duty passed. The party has been disbanded and the duty ended." and I was sent back to the open world. My pre-made party was disbanded without any notice because I was not in a party at that moment.

That's it!

Hope SE can fix this weird bug ASAP. Thanks.