Reproduction scenario:
Post a message with some accents.
Exemple: Voilà un café
Now quick edit the message:
The text appears with accent remplaced by double characters : Click to view attachment
If you validate the quick edit at this stage, the post is now displayed with this doubles characters instead of accents.
If you you enter some text that contains accents before completing the quick edit, it appears that the new accents are correctly displayed, while existing accent are displayed with this "double char"
Now if you Full Edit the message:
The first accents (the one replaced by double char) are then correctly displayed in the editor while the second ones (the ones added in quick edit) are replaced by question marks :
Voil� un caf�
Here are some screenshots:
With "question marks"

With "double chars"