Thunderbird Rendering UTF-8 Emails Incorrectly

I imported all of my Outlook emails into Thunderbird. Since then, I've noticed that many of the emails render incorrectly - for example - reason appears as reason�.

Inspecting the source, these issues seem to happen due to UTF-8 encodings where hidden characters (such as reason=85 - as opposed to just reason) were included when the email uses Content-Type: text/html; charset=UTF-8. I'm able to use Charset Menu to "repair" these emails, but it only works on an individual email, and doesn't re-save the email.

Is there a way to get Thunderbird to properly render all of these emails?