The worst part of the bug is that it screws up command-recall-and-edit. Any non-ASCII character in the command has a bogus space displayed after it. But the space is counted when the cursor passes it, resulting in the edit position not being where the cursor is.
If I copy a line with such a bogus space and paste it in TextEdit, the space goes away. So I don't think it's a bug in the shell. Also, it happened in both bash and zsh.