I wanted to make sure you're aware of a problem with Canvas inbox. An email was sent about this a while back, but it's important enough for a reminder to be sent. If you send a message to multiple people in Canvas, every response to that thread from then on will go to all of the original recipients (even if you don't see all of the names in the To box).
Here is an example of something we've seen:
An instructor sends an email to a student and a grading assistant. Weeks later, the grading assistant emails back (in that same thread) and asks the instructor if they have recently been in touch with that student. The student's name is likely not obviously included in the latest email. The instructor then gives details about the situation to the grading assistant that they may not want the student to see. The student does see it because they were originally included in that thread.
We have seen multiple examples of this happening. It's always a good idea to start a new thread for any message that you don't want anyone else to see.