The only possible way to achieve that is to mix files from different repositories which were using the same password (The master key of a repository never changes, and restic does not modify the config file, except in the moment when a repository is upgraded to version 2). There might also be some way to achieve that when a bitflip occurs in memory while creating a new key (although newer restic versions should also detect that). But that definitely cannot happen when calling s3 sync.
Judging from the error message it seems likely that the data and snapshots might use keyA. You could create a fake config file by following the steps in Changing a repository id - #6 by MichaelEischer . But make a backup copy of the current config file and the keys/ folder first. Additionally, the fake config file will allow you to access existing data (assuming that keyA works), but it will likely break deduplication for newer data.