I am able to reproduce, but I'm not sure this is a bug. You are adding an Ext field container into a standard HTML form. I found other field options that didn't function as expected, and I suspect that an HTML form is not set up to handle features provided in Ext-rendered form elements. Your field container behaves just fine when rendered inside an Ext.form.panel. What is the use case for generating your form with HTML tags?
I have a older code which mixes PHP and HTML that I want to plugin some extjs components, thats my use for a standard HTML page.
About the checkbox, I think it is a bug, because ExtJS is not creating the name property for the checkbox element, as it is creating for the nuumberfield, which get submitted normally. Is you inspect the DOM, you will not find the name anywhere for the checkbox, but for the numberfiled, it is there: