OP here. Apologies for not providing the real world example that triggered the behavior but here's some more background: We have some make tasks that set up a local dev environment and rely on environment variables to control a few things. We ran into an issue where if someone started another tmux session and then ran the make task to start the dev environment the env vars wouldn't be set correctly causing some confusing behavior.

Ultimately we decided to use a named tmux socket to ensure the environment variables were picked up correctly when set via the make task but you can also use `set-environment` as well if the ergonomics hit of having to use the named socket each time is too much. It's nice that tmux provides a few work arounds but I thought the original behavior was not intuitive.