Thank you for your time digging into the cause of the issue.
We investigated the issue and we managed to pin down it in our Checkbox
widget. Generally speaking, "on" value is returned when there is no value attribute set to the input element. However, there is an issue in the rendering of the component that interfere the proper value setting and thus removes the value attribute leading to the observed behavior.
I logged it in our public github repo and we already pushed a fix in our code. You could track the progress of the issue here: https://github.com/telerik/kendo-ui-core/issues/4215
which is already in Ready For Test stage. The fix will be included in our next Latest Internal Build
and in 2018 R2 SP1