I am having the exact same issue with existing customers. I also used to be able to customize the email addresses in the invoice before sending to different PM's within the same customer organization. Now it will let me change the email addresses on the invoice but when I save and send it, it defaults back to the original email address. I am so frustrated with this issue and don't understand why no one is working on fixing it instead of giving the generic suggestion to report it. It has obviously been reported now just fix it!!!