Steps to reproduce the issue

1. Create a custom field of the type checkboxes and assign two options, one of them with a 0 value. Eg:
2. Select only the 0 value ("No" in my example) and save
3. Reopen the form and check the value. Check the item in frontend.

Expected result

The selected 0 value is shown in both places

Actual result

The selected 0 value is ignored in both places

Additional comments

This is an issue with how com_fields sets up the fields.
If I specify a checkboxes field in the form XML with the same options, it works fine. So it's not the JFormFieldCheckboxes that is buggy.
When debuging I saw that when the form is defined in the XML, the value in https://github.com/joomla/joomla-cms/blob/staging/libraries/joomla/form/fields/checkboxes.php#L153 is an array (even with only one option) and the $hasValue passes. However if it is coming from com_fields, the value is a string if it's only one selected option and thus the check fails.

Now we could adjust the formfield to take care of strings as well, but I don't know the side effects as checkboxes are really a strange thing to handle.

I'd prefer to fix it in com_fields, but I haven't found yet where we could do it.
Also it may be related to the $forceMultiple property that is set in the checkboxes field. Maybe com_fields need to read that one and act accordingly.