When you attempt to enter an address in the cc: field focus jumps back to the To: field and overwrites any adresses there. Only way to cc someone is to enter it in the To: field separated by a comma. Ubuntu 12.04 LTS, kernel 3.2.0-60-generic-pae
Are you sure that it's not just that the original To: field has simply scrolled out of the address entry section? That's usually what this bug turns out to be.
see comment #2 and make that area of the window larger.