Edit: I couldn't re-create the issue. Are you sure you only changed the formation? Was it in the mermaid dungeon or one of the other adventurer maps?
It was in Mermaid dungeon yes. I just moved the original leader
and move Grey as target so it'd switch them. I had finished mermaid dungeon by then already. I can't reproduce it with the other two presuming due to their identity in game.
Edit: I did finish pretty much everything related to the female adventurer group prior to that.
I tried reproducing it, I failed a few time. I'm not exactly sure how I did that now. Far I can remember, this was before wedding are done. I had everyone's rings but Mira's was done, but I didnt propose to anyone beside Mira, whose wedding is already finished, yet, and this was right after the cutscene with Ash, I went straight to unlock Grey's further event and headed to the mermaid dungeon finished it and did several farming and upgraded MDEF for
I have a save to provide that was a while ahead of the bug event.
Edit3: I changed the equipment too it's possible an id for one of these influenced the code. Grey had White Silver Blade, Heavy Katryx Helmet, Adventurer Chest Plate, Adventurer Amulet, Talisman of Regeneration, and Adventurer Shoes.
Edit4: It's entirely possible that I did the Wedding for
first before starting Mermaid dungeon, but I have short term memory and I spent hours on hours of playing this yesterday so I'm hazy on what I did in chronological.
Edit5: Spending some time trying to figure out things with my bug save in the mermaid dungeon ended up fixing it anyway.