As described in tmux-plugins/tpm#295 and tmux/tmux#4166, run-shell is now setting $SHELL to sh which causes tmux-sensible, at line 103, to incorrectly set default-command to use 'sh' instead of the user-selected shell. This means new panes and windows will fire up with sh instead of what the user expects (e.g., zsh).

A work around is to put set -g default-command "reattach-to-user-namespace -l $SHELL" directly into ones .tmux.conf to override tmux-sensible.

One possible fix would seem to be to modify tmux-sensible to use the tmux default-shell variable instead of $SHELL.

Or see this comment for other suggested fixes.