I noticed two mistakes:

1. The Select modal view of the type "checkboxes" shows near to the top right corner a green (-). But when you click on this, then a new selector is adding. In my opinion this must better be a green plus (+):
   

2. On a field type "checkboxes", when you have a value = 0, then this checkbox can never be choosen. See the following steps to reproduce this bug:

Steps to reproduce the issue

1. Install joomla from staging fresh, with some example articles (on the installer, have choosen the last option).
2. Login as administrator (on the back end)
3. Navigate to Content > Fields
4. Click New.
5. Choose type "checkboxes"
6. Set the title to (for example): Hardness
7. Click on select (last field) and add the following three selectors (or options):
   Text, Value
   Easy, 0
   Medium, 1
   Powerful,2
8. Save the created selectors and save the field.
9. Opens an article and navigate to the tab fields
10. You can see now three checkboxes.
11. Click/set/check into the first checkbox.
12. Save the article.

Expected result

The checkbox should be set.

Actual result

The checkbox is always empty/unset.

System information (as much as possible)

Source code synchronized from staging branch
OS Windows
PHP 7.0.13
MySQLi 5.5.5-10.1.19-MariaDB

Additional comments

From my point of view, the value "0" may be the problem. The other checkboxes in the described example work correctly.