I am extremely frustrated because the issue has not been resolved. After reaching out to technical support, I was advised to provide my input on the following problem:

Whenever I create a message on Invoice template "A", it changes the message on all other templates "B" & "C". The same applies to the email subject and body assigned to the template. I was told, "Currently, this is how QBO is being programmed. It only has 1 default for messages."

It is essential for me to have different invoice body messages and associated email messages on each template! Otherwise, what is the point of creating individual, separate templates? Currently, it is counter productive.