Weird Vi-mode behavior

[Matthieu Louvel de Monceaux <glory69@gmail.com>]

Hello,
I'm using zsh 4.3.9+1101.
On bash i was used to work with the vi-mode activated.
I activated it in zsh by

bindkey -v


And now it works, except one huge thing :
When we return from command mode to insert mode, we can't modify
the text from the beginning to the spot where we got insert ...
Definitely a weird behavior ...

To get it too, just type 'foo folk x' for example.
Then Esc, type 'b' to go to the previous word, then press 'i' to go 
insert mode.
Now you can't edit the 'foo ' part anymore.

Thank you by advance,
M. Louvel de Monceaux