Ye that is not supported, if you need it like that, you can make the following, the checkbox internal value can be 1, display value nothing.
Then, as not checked boxes by default send nothing at all,
1) go to phpmyadmin
2) locate your database, go to VL_sites tables
3) on that table click "structure" at the top
4) lets say you named your field "my_new_field"
4.1) locate column "acf_my_new_field" and "acf_my_new_field_display"
4.2) edit the first one and give it a default value of "0"
4.3) edit the second one and either keep it blank or give it a default value of "Not set" or so
This way you could simulate your desired effect.
If they check the field, it would "1" in the database and if not it would be "0" since that is the columns default value
That should work, but i havent tested it, so you are my labor rabbit