I'm creating a numeric text box with data attributes for the initialization but it does not seem to be working, can someone help to explain why or is this just a Kendo UI bug please?
I don't think you can mix the usage of data- attributes, and javascript kendo widget creation.
You need to either create the object using javascript with all options set, or use data- attributes to define the widget and options, and a kendo.bind to create them. See sample.