Tracing module update for inheritance in views

I'm trying to manipulate the `sale` form by a module with inheritance in views. I have following XML for testing:sale.order.form.custom.120sale.order AFAIK that should remove the quantity field from the form. But it won't. I tested the XPath expression wit a query tool. Should be fine. Is there any debug/trace level to see what happens, when reading this XML to update the view ? (Updates removed, as incorrect)