This is a game breaking bug, also happened to me. Since RPG Maker MV is essentialy JavaScript, I've figured out how to solve this with the help of Gemini, but it involves loading code workarounds on Terminal to prevent the game to breaking when switching parties. It happens because group 2 has characters with "null" value and then the Yanfly's Plugin responsible for this breaks. So it basically involves loading one code sequence to prevent the game from breaking, then another code injection to clean the NULL characters on group 2. Then, another code to manually add the characters that were supposed to be in the party. After cleaning the values, I had only Vera and Nobumasa. Darius and Elina were supposed to be in group 2. Unfortunately, this manual addition of characters involves finding the Actor ID of your character, and this can bring some spoilers and also allows you add characters that weren't supposed to be there... After adding, there is also a fourth code injection that will prevent the game from breakign again (If I fix and switch, and then try to switch back the game breaks).
I've fixed this only as a way to escape this damndungeon.