I missed the part where you mentioned Save Format, but that also means you’re using ACF4. Sorry for the oversite.
If you want to be able to sort by this field then you need to set the save format to something that can be sorted. To sort by a date in WP the format must be YYYY-MM-DD. to sort by but number or text the saved value still needs to be similar. Year, Month, Day.
This is likely one of the reasons why the save format was removed from ACF5.
