The reason it is hard to fix for all campaigns/templates at the same time is because there is no way (in the code) to detect that one of the emails should be encoded or not.
As you saved some campaigns after the update, part of your emails are encoded, and part of them aren't (so we can't run the update script again, it would break the special characters in the newly created / saved emails).
There is a way to re-run the update script but we didn't add any easy way to do it as some of the actions in it are not meant to be processed twice.
If you didn't create new emails (or if you don't mind breaking the ones you recently saved), you can edit the AcyMailing configuration page, press F12 and change any input name into config[version] in the "Elements" tab of the appearing area.
Once done, you can change the AcyMailing version to 6.1.5, save the configuration then install again the v6.1.6 package.
If you do this, I highly recommend you to make a backup of your data in case the result isn't correct.
