Tmux hampers osascripts display notification
I don't plan to use Tmux but just from curiosity I leafed through its manual pages and saw under "Options":

focus-events [on|off]
When enabled, focus events are requested from the terminal if supported
and passed through to applications running in tmux. Attached clients 
should be detached and attached again after changing this option.


You might look into Tux' options to get what you want, I guess.