This is urxvt without tmux

This is urxvt with tmux and utf8 not enabled in the config

This is urxvt with tmux and utf8 enabled in the config

How can I make tmux act consistent with the initial result?
Would you please try the following and see if it helps?
Clarification:
I have seen the existence of ~/.tmux.conf may cause problem so just for testing I suggested to rename the file. And forcing LANG=en_US.UTF-8 is another measure to ensure the correct locale to be used (since Hal didn't supply his LANG setting(s))