Example:

My family members send me a group email with a few CCs (all family members) and I respond.

My friend, Joe, emails me and his email shows up in the thread of email that my family has sent me previously.

That's happened with 2 of my contacts. Emails from people who were not included in the CCs of my family's emails show up in the wrong place. I have since signed out and restarted my laptop but the same thing keeps happening.