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There is now an item in the Options -> Account Preferences screen for you to select how the default From address is filled in when you reply to a message. The new option is in the Compose section of the screen and is called 'From when Replying'.
If you select 'Best personality', then the From address of the new message will be the personality which seemed most appropriate, judging from the headers of the original message. If none seem appropriate, then your default personality will be used.
If you select 'Always original' then the From address of the new message will be the address that the original message used to reach you, even if it does not match any of your personalities.