This was resolved by reinstalling the game. Happy to say that this didn't affect my saves (I didn't try reinstalling straight away because I was afraid to lose progress).
Solved! Sorry for not checking every option before posting.
In case anyone else runs into this: I hadn't realised that character progression was saved separately and completed adventures are saved with the characters. When I made a new party with the experienced characters it essentially became a copy of the old one so I didn't lose anything.
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