tmux 2.8 contains a change in behaviour compared to previous versions when
the current directory differs from the contents of the PWD environment
variable:

Consider running the following at a shell prompt in a tmux session:

 cd /
 PWD=/etc tmux split-window

With tmux 2.7 (Leap 15.0) the shell in the new split ends up with its
current directory set to /, whereas with tmux 2.8 the shell in the new
split ends up with its current directory set to /etc.

Unfortunately, this breaks processes (such as openembedded's terminal.py)
which change the current directory but don't change PWD.

The problem was reported upstream at https://github.com/tmux/tmux/issues/1549
and it has been fixed.