Issues with Unicode symbols in some applications running in Terminal.app and iTerm2
Interesting question! I did some tests on my side. 

zsh

xXx:~/ $ &lt;2691&gt;                                                                                                                                                                                              
zsh: command not found: \u2691
xXx:~/ $                                                                                                                                                                                                                                                                                         
zsh: command not found: 
xXx:~/ $ ℃                                                                                                                                                                                                                                                                                                                                                 
zsh: command not found: ℃


I did also several tests with other shells and results are not the same: 

sh

sh-3.2$ ⚑
sh: ⚑: command not found


bash

Ok, that should be the same as sh:

bash-3.2$ ⚑
bash: ⚑: command not found


csh/tcsh

[LeBouc-xXx:~] xXx% \U+2691
⚑: Command not found.




Apparently, all that shells are not set with the same input/output options. I tried with xterm/terminal and iTerm2 to see if I could be a software dependant problem. That's the same everywhere.

I don't have the time to search/read that but you might find some useful informations about the zsh line editor here.

If you find it, keep me up-to-date, I would be pleased to have the final answer to your question.