Your Tmux working directory is the root directory of the session. You can figure out what it is by opening a new window or pane in your session. The directory you start in is your working directory.
Sometimes it’s not the best directory for the type of project you’re developing. For instance, it could be set to the root directory of an umbrella app, when you’re working exclusively in one of the subdirectories.
Reset it with the
$ tmux attach-session -t my_session -c ~/my_project