I noticed this too.
In fact, if you uncheck the "enable IPV6" checkbox then close the
EDIT dialogue, if you open up the EDIT dialogue on that interface
again the IPV6 checkbox it set again even though you disabled it.
In fact, if you uncheck IPV6 with DHCP disabled, it says you must
enter an IPV6 address yet this is impossible because the IPV6 address
entry field is shadowed out since "enable IPV6" is unchecked.
*** This bug has been marked as a duplicate of 206148 ***