I've often felt a bit abnormal for doing this. I guess I only ever see each person in one mode, so I had no idea if other people do it or not. Nice to know at least some other people do.
It's also something that's been much remarked-upon by various minority groups. I've heard a lot of African-American comedians, friends, and commentators mention how they have feel the need "act white" when they're not around other black people. You hear the exact same thing from LGBT folks as well - they often choose to "act straight" by default in a lot of environments.
I've seen this a lot firsthand myself. It's also a good reminder that having the option to code switch is a real privilege. For some people it's not a coping skill, it's a literal survival tactic.